Docs for target-specific commands

[Eli Zaretskii]

I notice that some targets define commands private to them, which are not 
documented anywhere in the manual.  The only ones which are documented 
are for remote debugging of certain targets.  However, most commands 
specific to native debugging on certain platforms are undocumented.

Should we create a chapter in the manual where target-specific commands 
would be documented, with sections devoted to each target?