TLSRecv clarifications

Here is a round of clean-ups triggered by @ediosyncratic's comments about whether SSL_read() can return partial reads.

My verdict is that yes this is legal (and as a result it is legal for TLSRecv() as well), and this will be the case if the opposite side did an SSL_write() of smaller size than what we are reading, or smaller than TLS record size (16KB).

But given that we are using SSL_MODE_AUTO_RETRY and that our SendTransaction() never sends more than 4KB of payload (which is smaller than the TLS record size), ReceiveTransaction() should never get partial transaction payload.
The the comment should say "any other return means success" rather than "(also currently returns 0 for success but don't count on it)", which is the confusing thing I'm commenting on.  If you really want to mention that the present success code is 0, "any other return (currently only 0, but don't count on it) means success".