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.
Canonical use cases include filesystem cleanup (for the data in volumes that survives restart) and custom restart abort logic. For these cases, PostRestart is less usable and more messy and racy than PreRestart.