Add the missing RETURN VALUES section.

Add the missing RETURN VALUES section.
Triggered by OpenSSL commit 1f13ad31 Dec 25 17:50:39 2017 +0800
by Paul Yang, but reworded for intelligibility and precision.

While here, also expand the description of the "ret" argument of
BIO_callback_fn().  That's a fairly complicated and alarmingly
powerful concept, but the description was so brief that is was
barely comprehensible.