mprotect.2: Remove legacy BSD text

[brooks]

Summary

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

[brooks]

Per the discussion in D30442 this removes the incorrect description of mprotect granularity. Documentation of large pages is left for later.

[alc]

Ultimately, I think that the text will have to grow a new paragraph: (1) mentioning the possible existence of large page sizes on some architectures (See getpagesizes()); (2) that if the specified range does not cover an entire large page, the system will either demote the page mapping to a sufficiently small size or return an error, depending on whether the large page was created automatically or explicitly via shm_open(). In particular, I see no way of describing this without introducing the notion of demotion.