Emacs provides a variety of built-in help functions, all accessible to the user as subcommands of the prefix C-h. For more information about them, see Help in The GNU Emacs Manual. Here we describe some program-level interfaces to the same information.
This function finds all meaningful symbols whose names contain a match for the apropos pattern pattern. An apropos pattern is either a word to match, a space-separated list of words of which at least two must match, or a regular expression (if any special regular expression characters occur). A symbol is meaningful if it has a definition as a function, variable, or face, or has properties.
The function returns a list of elements that look like this:
(symbol score function-doc variable-doc plist-doc widget-doc face-doc group-doc)
Here, score is an integer measure of how important the symbol seems to be as a match. Each of the remaining elements is a documentation string, or nil, for symbol as a function, variable, etc.
It also displays the symbols in a buffer named *Apropos*, each with a one-line description taken from the beginning of its documentation string.
If do-all is non-nil, or if the user option apropos-do-all is non-nil, then apropos also shows key bindings for the functions that are found; it also shows all interned symbols, not just meaningful ones (and it lists them in the return value as well).
The value of this variable is a local keymap for characters following the Help key, C-h.
This symbol is not a function; its function definition cell holds the keymap known as help-map. It is defined in help.el as follows:
(define-key global-map (string help-char) 'help-command) (fset 'help-command help-map)
The value of this variable is the help character—the character that Emacs recognizes as meaning Help. By default, its value is 8, which stands for C-h. When Emacs reads this character, if help-form is a non-nil Lisp expression, it evaluates that expression, and displays the result in a window if it is a string.
Usually the value of help-form is nil. Then the help character has no special meaning at the level of command input, and it becomes part of a key sequence in the normal way. The standard key binding of C-h is a prefix key for several general-purpose help features.
The help character is special after prefix keys, too. If it has no binding as a subcommand of the prefix key, it runs describe-prefix-bindings, which displays a list of all the subcommands of the prefix key.
The value of this variable is a list of event types that serve as alternative help characters. These events are handled just like the event specified by help-char.
If this variable is non-nil, its value is a form to evaluate whenever the character help-char is read. If evaluating the form produces a string, that string is displayed.
A command that calls read-event, read-char-choice, or read-char probably should bind help-form to a non-nil expression while it does input. (The time when you should not do this is when C-h has some other meaning.) Evaluating this expression should result in a string that explains what the input is for and how to enter it properly.
Entry to the minibuffer binds this variable to the value of minibuffer-help-form (see Definition of minibuffer-help-form).
This variable holds a function to print help for a prefix key. The function is called when the user types a prefix key followed by the help character, and the help character has no binding after that prefix. The variable’s default value is describe-prefix-bindings.
This function calls describe-bindings to display a list of all the subcommands of the prefix key of the most recent key sequence. The prefix described consists of all but the last event of that key sequence. (The last event is, presumably, the help character.)
The following two functions are meant for modes that want to provide help without relinquishing control, such as the electric modes. Their names begin with ‘Helper’ to distinguish them from the ordinary help functions.
This command pops up a window displaying a help buffer containing a listing of all of the key bindings from both the local and global keymaps. It works by calling describe-bindings.
This command provides help for the current mode. It prompts the user in the minibuffer with the message ‘Help (Type ? for further options)’, and then provides assistance in finding out what the key bindings are, and what the mode is intended for. It returns nil.
This can be customized by changing the map Helper-help-map.
This variable holds the name of the directory in which Emacs finds certain documentation and text files that come with Emacs.
This function returns the name of the help buffer, which is normally *Help*; if such a buffer does not exist, it is first created.
This macro evaluates body like with-output-to-temp-buffer (see Temporary Displays), inserting any output produced by its forms into a buffer specified by buffer-or-name, which can be a buffer or the name of a buffer. (Frequently, buffer-or-name is the value returned by the function help-buffer.) This macro puts the specified buffer into Help mode and displays a message telling the user how to quit and scroll the help window. It selects the help window if the current value of the user option help-window-select has been set accordingly. It returns the last value in body.
This function updates the cross reference data in the *Help* buffer, which is used to regenerate the help information when the user clicks on the ‘Back’ or ‘Forward’ buttons. Most commands that use the *Help* buffer should invoke this function before clearing the buffer. The item argument should have the form (function . args), where function is a function to call, with argument list args, to regenerate the help buffer. The interactive-p argument is non-nil if the calling command was invoked interactively; in that case, the stack of items for the *Help* buffer’s ‘Back’ buttons is cleared.
See describe-symbols example, for an example of using help-buffer, with-help-window, and help-setup-xref.
This macro defines a help command named fname that acts like a prefix key that shows a list of the subcommands it offers.
When invoked, fname displays help-text in a window, then reads and executes a key sequence according to help-map. The string help-text should describe the bindings available in help-map.
The command fname is defined to handle a few events itself, by scrolling the display of help-text. When fname reads one of those special events, it does the scrolling and then reads another event. When it reads an event that is not one of those few, and which has a binding in help-map, it executes that key’s binding and then returns.
The argument help-line should be a single-line summary of the alternatives in help-map. In the current version of Emacs, this argument is used only if you set the option three-step-help to t.
This macro is used in the command help-for-help which is the binding of C-h C-h.
If this variable is non-nil, commands defined with make-help-screen display their help-line strings in the echo area at first, and display the longer help-text strings only if the user types the help character again.
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https://www.gnu.org/software/emacs/manual/html_node/elisp/Help-Functions.html