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).
- Rearrange 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.
Requested by: wblock (1), kib (4)
Submitted by: wblock (2)