Fix return value of `defun' and un-define it.

Fix return value of `defun' and un-define it.
* src/data.c (Fdefalias): Return `symbol'.
* doc/lispref/functions.texi (Defining Functions):
* doc/lispref/macros.texi (Defining Macros): Un-define the return value of
`defun', `defmacro' and `defalias'.

Fixes: debbugs:11686