Small improvements in comments in async.c.

We seem to have lost a line somewhere along the way in the comment block
that discusses async.c's locks, because it suddenly refers to "both locks"
without previously having mentioned more than one.  Add a sentence to make
that read more sanely.  Also, refer to the "pos of the slowest backend"
not the "tail of the slowest backend", since we have no per-backend value
called "tail".