aio/README.md comments

aio/README.md:

* In the section "### IO can be started in critical sections", the
first paragraph seems like it belongs in another section.

* The README generally mixes design goals with implemented
functionality. For instance, we're only using it on the read path
currently, but the README mentions WAL writing several times. We should
probably clarify that a bit.

* "`io_method=sync` does not actually perform AIO but allows to use the
AIO API while performing synchronous IO. This can be useful for
debugging." Sync is still useful for cases where the shared buffers are
a small fraction of system memory, right?

* "Particularly on modern storage..." I assume this is talking about
SSDs, but it could also mean some kind of network block storage. If our
architecture is changing in response to new real-world hardware, we
should briefly try to connect the design choices to assumptions about
hardware, where appropriate.

Regards,
    Jeff Davis