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(elisp)Function Documentation

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Documentation Strings of Functions
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   A lambda expression may optionally have a "documentation string" just
after the lambda list.  This string does not affect execution of the
function; it is a kind of comment, but a systematized comment which
actually appears inside the Lisp world and can be used by the Emacs help
facilities.  Note: Documentation, for how the DOCUMENTATION-STRING is
accessed.

   It is a good idea to provide documentation strings for all the
functions in your program, even those that are called only from within
your program.  Documentation strings are like comments, except that they
are easier to access.

   The first line of the documentation string should stand on its own,
because `apropos' displays just this first line.  It should consist of
one or two complete sentences that summarize the function's purpose.

   The start of the documentation string is usually indented in the
source file, but since these spaces come before the starting
double-quote, they are not part of the string.  Some people make a
practice of indenting any additional lines of the string so that the
text lines up in the program source.  _This is a mistake._  The
indentation of the following lines is inside the string; what looks
nice in the source code will look ugly when displayed by the help
commands.

   You may wonder how the documentation string could be optional, since
there are required components of the function that follow it (the body).
Since evaluation of a string returns that string, without any side
effects, it has no effect if it is not the last form in the body.
Thus, in practice, there is no confusion between the first form of the
body and the documentation string; if the only body form is a string
then it serves both as the return value and as the documentation.