aio_suspend.3: Various additions and rewordings.

aio_suspend.3: Various additions and rewordings.

Give some arguments more meaningful names.
More explicitly describe the 'nitems' argument.
Explicitly note that return is immediate if an I/O operation
has already completed.
Note that aio_error(3) should be used to scan the aiocb list
after a successful return.
Add references to other relevant pages.
Various other pieces rewritten.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>