A documentation string is written using the Lisp syntax for strings, with double-quote characters surrounding the text. It is, in fact, an actual Lisp string. When the string appears in the proper place in a function or variable definition, it serves as the function's or variable's documentation.
In a function definition (a
defun form), the
documentation string is specified after the argument list, and is
normally stored directly in the function object. See Function Documentation. You can also put function documentation in the
function-documentation property of a function name
(see Accessing Documentation).
In a variable definition (a
defvar form), the documentation
string is specified after the initial value. See Defining Variables. The string is stored in the variable's
Sometimes, Emacs does not keep documentation strings in memory.
There are two such circumstances. Firstly, to save memory, the
documentation for preloaded functions and variables (including
primitives) is kept in a file named DOC, in the directory
doc-directory (see Accessing Documentation).
Secondly, when a function or variable is loaded from a byte-compiled
file, Emacs avoids loading its documentation string (see Docs and Compilation). In both cases, Emacs looks up the documentation string
from the file only when needed, such as when the user calls C-h
describe-function) for a function.
Documentation strings can contain special key substitution sequences, referring to key bindings which are looked up only when the user views the documentation. This allows the help commands to display the correct keys even if a user rearranges the default key bindings. See Keys in Documentation.
In the documentation string of an autoloaded command (see Autoload), these key-substitution sequences have an additional special effect: they cause C-h f on the command to trigger autoloading. (This is needed for correctly setting up the hyperlinks in the *Help* buffer.)