bug#22813: 24.5; doc string of `apropos-variable' - mention C-u

[Drew Adams]

Subject line says it all.  The doc string says something
about Lisp argument DO-NOT-ALL, but it does not speak to an interactive
user, telling her that C-u gives that behavior.



In GNU Emacs 24.5.1 (i686-pc-mingw32)
 of 2015-04-11 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/usr --host=i686-pc-mingw32'