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>

docs/misc/xen-command-line.markdown

@@ -740,13 +740,18 @@ Specify which console gdbstub should use. See **console**.
 ### gnttab\_max\_frames
 > `= <integer>`
 
-Specify the maximum number of frames per grant table operation.
+> Default: `32`
+
+Specify the maximum number of frames which any domain may use as part
+of its grant table.
 
 ### gnttab\_max\_maptrack\_frames
 > `= <integer>`
 
-Specify the maximum number of maptrack frames domain.
-The default value is 8 times **gnttab\_max\_frames**.
+> Default: `8 * gnttab_max_frames`
+
+Specify the maximum number of frames to use as part of a domains
+maptrack array.
 
 ### gnttab\_max\_nr\_frames
 > `= <integer>`