Remove _POSIX_PRIORITIZED_IO references from man pages

We don't support it, so there's no need to tell readers what would
happen if we did. Also, don't remind the user that a certain field is
ignored by aio_read. Mentioning every ignored field would make the man
pages too verbose.

MFC after: 1 week
Sponsored by: Axcient
Reviewed by: Pau Amma <pauamma@gundo.com>
Differential Revision: https://reviews.freebsd.org/D42622