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

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

GDB's execution commands have a foreground and background variant: f.i.,
there's 'continue' and 'continue&', and both are listed individually in the
'Command, Variable, and Function Index'.  But the '&' is not listed in the
'Concept Index' as being connected with the concept background execution.

This patch adds an '&' in the 'Concept Index':
...
 * $_, $__, and value history:            Memory.             (line  119)
+* &, background execution of commands:   Background Execution.
+                                                             (line   16)
 * --annotate:                            Mode Options.       (line  121)
...
pointing to this line in 'Background Execution':
...
   To specify background execution, add a '&' to the command.
...

Build on x86_64.

2018-06-14  Tom de Vries  <tdevries@suse.de>

	* gdb.texinfo (Background Execution): Add @cindex for '&'.