docs: sysctl/fs: merge the aio sections

There are two sections documenting aio-nr and aio-max-nr, merge them.
I kept the second explanation of aio-nr, which seems clearer to me,
along with the effects of the values from the first section.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Link: https://lore.kernel.org/r/20220930102937.135841-4-steve@sk2.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>