[RFA/doco] Document "set/show multiple-symbols" command

[Eli Zaretskii]

> Date: Thu, 3 Apr 2008 14:59:07 -0700
> From: Joel Brobecker <brobecker@adacore.com>
> Cc: gdb-patches@sourceware.org
> 
> 2008-04-03  Joel Brobecker  <brobecker@adacore.com>
> 
>         * NEWS: Mention new commands set/show multiple-symbols.
> 
> OK to commit?

Yes, thanks.