MFC r349239, r349241: update vm_map_protect.9

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.

Clarify vm_map_protect max_protection downgrade

As reported in review D20709 by brooks calling vm_map_protect to set a
new max_protection value downgrades existing mappings if necessary (as
opposed to returning an error).

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