[gdb] Add 'Concept Index' entry '&' for background execution

[Eli Zaretskii]

> Date: Thu, 14 Jun 2018 20:53:32 +0200
> From: Tom de Vries <tdevries@suse.de>
> 
> +@cindex @code{&}

Please use this instead:

  @cindex @code{&}, background execution of commands

This will allow users to understand more quickly what this entry is
about.

Thanks.