munmap.2: Add STANDARDS and note about portability

POSIX used to specify that munmap shall fail with EINVAL if the addr
argument is not a multiple of the page size, but that was changed to
may fail. Note that we conform to contemporary POSIX and include a
brief note for portable programs.

Reviewed by: brooks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48481