MFC 304476: Fix various nits in the aio operation manpages.

MFC 304476: Fix various nits in the aio operation manpages.

- Avoid double use of "request" in a single sentence.  Instead, describe
  aio_sigevent as being used to request notification of the associated
  operation's completion.  This matches the language used to describe
  aio_sigevent in aio(4).
- Simplify the prohibition on modifying buffers while requests are in
  flight.
- Fix case mismatch.
- Drop note about not using stack variables. C programmers should be able
  to figure out if a stack variable is safe based on the later warning
  about the life cycle requirements of control blocks.
- Remove prohibition on modifying the I/O buffer for aio_fsync() since
  it does not use an I/O buffer.  For aio_mlock(), prohibit modifications
  to the mapping (e.g. due to mprotect, munmap, mmap, etc.) but do not
  prohibit modifications to the memory backing the buffer (stores into
  the pages backing the buffer).

git-svn-id: svn://svn.freebsd.org/base/stable/9@304617 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f