x86/pv: Fix `global-pages` to match the documentation

x86/pv: Fix `global-pages` to match the documentation

c/s 5de961d9c09 "x86: do not enable global pages when virtualized on AMD or
Hygon hardware" in fact does.  Fix the calculation in pge_init().

While fixing this, adjust the command line documenation, first to use the
newer style, and to expand the description to discuss cases where the option
might be useful to use, but Xen can't account for by default.

Fixes: 5de961d9c09 ('x86: do not enable global pages when virtualized on AMD or Hygon hardware')
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>