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.
We should just omit PostRestart for now. Whatever the user needs to do, they can do in PostStart.

Let's wait until we get some user input before we start to speculatively add lots of questionable hooks. PostStart and PreStop are enough to establish the pattern.