gdb/doc: add some index entries relating to mi-async setting

gdb/doc: add some index entries relating to mi-async setting

I noticed that the mi-async setting was not referenced from the index
in any way, this commit tries to rectify that a bit.

The @cindex lines I think are not controversial, these same index
entries are used elsewhere in the manual for async related topics (see
@node Background Execution).

The only bit that might be controversial is that I've added a @kindex
entry for 'set mi-async' when the command is documented as '-gdb-set
mi-async' (with a similar difference for the show/-gdb-show).

My reasoning here is that nothing else is indexed under -gdb-set or
-gdb-show, and as -gdb-set/-gdb-show are just the MI equivalent for
set/show anything that is documented under set/show can be adjusted
using -gdb-set/-gdbshow, and so, I've tried to keep the index
consistent for mi-async.