Clarify that vm_map_protect cannot upgrade max_protection

It's implied by the man page's RETURN VALUES section, but be explicit in
the description that vm_map_protect can not set new protection bits that
are already in each entry's max_protection.

Reviewed by: brooks
MFC After: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20709