documentation request: explain that Transport.open_channel src_addr is merely advisory

[eichin]

Was attempting to set up a simple one-shot forward and forward.py looked a bit complex so I thought I could just work with the open_channel docstring. I was misled by the source address (in the direct-tcpip case specifically) because I assumed that it was actually used - if it actually said in the docstring that it was only so that remote server could produce more useful logs (an assumption on my part, but since it's user specified the server can't possibly be trusting it for anything, right?) I would have more quickly realized that setting up the listening socket was entirely my problem, and that this call isn't to set up forwarding, merely to ask for the far side of the connection to happen.

(In retrospect, it's "obvious" and the complexity in forward.py makes sense - but even after not finding any listening code in transport.py, it still took a while for "what was actually going on" to sink in, and it wouldn't take much of a hint to help out here. A real bonus would be explaining which calls handle which parts of the -L and -R openssh command line arguments, but just a sentence or two about src_addr would have saved me hours.)

#1410 is sort of related.