Add some documentation for the container environment.

@smarterclayton @bgrant0607 @thockin @erictune 

We've talked about some of this in the past.  Mostly this describes what is already implemented, but there are some proposals in there as well, please take a look.
/cc @thockin 

Docker also sends SIGTERM to containers, with notice. We should continue to support that, as many applications already handle clean shutdown via SIGTERM, since it is the traditional mechanism. 

The notice given is not currently configurable -- 10s last I looked, and is sent after completion of the PreStop handler. We should bite the bullet and explicitly support a configurable grace period, which should cover the time spent in the PreStop handler, if any, as well. Otherwise, if a user wanted to use SIGTERM to signal clean shutdown, which I expect to be common, and 10s weren't enough time, they'd need to write a PreStop handler that sent SIGTERM to the lead process (always PID 1?) and waited for it to terminate before exiting.