Update API documentation of transport send/recv functions

[aggarw13]

Update API of the transport interface functions for the following:

• Transport recv() should NOT block when requested to read a single byte. It MAY block for the underlying socket timeout when requested to read n > 1 bytes.
• Transport send() should NOT return an error when the send fails because of a full TX buffer of the underlying network stack, so that the calling library can retry the send operation.

[RichardBarry]

Here if bytesReceived contains an error code that just means the TCP read timed (or would block) then that is not an error so bytesReceived needs to be translated into whatever the calling function will interpret as "try again" - which for our code is zero, but if this function is calling the TCP read function and passing a value back to the TLS stack then zero may also be interpreted as an error.

[aggarw13]

Have updated the pseudo code block to add a check for a retriable error.

[RichardBarry]

Similar comment as per the receive - when the error is not really an error, rather just a full buffer, the error has to be translated into whatever the calling function interprets as meaning 'try again' until you time out.

[aggarw13]

Yes, the zero return value represents that the operation can be re-tried.