arch: x86: Better document that CR0.WP will also be set when CR0.PG is

Setting bit CR0.WP (bit 16) will inhibit supervisor threads from
writing to RO pages.  It's a necessary flag to be set, and the constant
name CR0_PAGING_ENABLE didn't reflect the fact that the 16th bit was
being set.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>