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).