Extended-remote fork event docs

Extended-remote fork event docs

This patch contains the accumulated documentation changes for the
rest of the extended-remote follow fork patchset.

gdb/ChangeLog:

        * NEWS: Announce fork support in the RSP and support
          for fork debugging in extended mode.

gdb/doc/ChangeLog:

        * gdb.texinfo (Forks): Note that fork debugging is
          supported in extended mode.
          (Remote Configuration): Add fork event features to table
          of packet settings.
          (Stop Reply Packets): Add fork events to list of stop reasons.
          (General Query Packets): Add fork events to tables of
          'gdbfeatures' and 'stub features' supported in the qSupported
          packet, as well as to the list containing stub feature
          details.