docs: correct descriptions of gnttab_max_{, maptrack}_frames

docs: correct descriptions of gnttab_max_{, maptrack}_frames

gnttab_max_frames incorrectly referred to numbers of grant tab
operations and gnttab_max_maptrack_frames was confusingly worded.

Add the default for gnttab_max_frames while here (it's currently the
same on all arches since no arch uses the available arch override) and
adjust the default for gnttab_max_maptrack_frames to match the normal
form.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>