Update documentation

[hessjcg]

Issue #397 comment spells out the operator's behavior in more detail. Add this to the docs.

Also, describe how #425 makes graceful shutdown work smoothly, including:

• exit 0 on sigterm
• admin /quitquitquit and preStopHook lifecycle handlers
• Using CSQL_QUIT_URLS envvar in jobs and cron jobs

Add documentation on how to use unix sockets #443

Add documentation on why namespace is not part of the workload selector #467