Two improvements: first, document that the container-sync process
connects to the remote cluster's proxy server, so outbound
connectivity is required.
Second, rewrite the behind-the-scenes container-sync example and add
some ASCII-art diagrams.
Fixes bug 1068430.
Bonus fix of docstring in wsgi.py to squelch a sphinx warning.
Change-Id: I85bd56c2bd14431e13f7c57a43852777f14014fb