MFC r303104, r303106:

Update to reflect the fact that pipe() is a wrapper around the pipe2()
system call.

Reviewed by: jhb, wblock
Sponsored by: DAPRA, AFRL

Change wording to use function rather than system call in the
description
as well.

Authored by: zeising
Reviewed by: brooks

Approved by: re (gjb)
Sponsored by: DAPRA, AFRL