mprotect.2: Remove legacy BSD text

This text dates to the BSD 4.4 import and is misleading. The mprotect
syscall acts on page granularity and breaks up mappings as required to
do so.

Note that with the addition of non-transparent superpages (aka
largepages) the size of a page at a given address may vary. This
commit does not attempt to address the lack of documentation of this
feature.

Sponsored by: DARPA

Reviewed by: alc, mckusick, imp, kib, markj
Differential Revision: https://reviews.freebsd.org/D31776