doc: posix: async-io: clean up async-io documentation

The async-io documentation was in need of some beautification.

Rather than repeating the same thing inside of the table, just
mention that the whole option is expected to fail enosys.

At this time, there does not seem to be sufficient need for
implementing async io, so it is only present so that conforming
applications can still link properly.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>