Various small changes in addition to the following.

Various small changes in addition to the following.
(What Is a Function): `functionp' returns nil for macros.  Clarify
behavior of this and following functions for symbol arguments.
(Function Documentation): Add `\' in front of (fn @var{arglist}) and
explain why.
(Defining Functions): Mention DOCSTRING argument to `defalias'.  Add anchor.
(Mapping Functions): Add anchor.  Unquote nil in mapcar* example.