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.
Don't qualify "environment variables", which has a well-known meaning, with "key-value".  It makes it sound like they are different from regular environment variables.