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.
The amount of surgery necessary to make this work isn't worth it in this revision.  We'll wait for better support or some further revision of this api.