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.
As described in https://github.com/GoogleCloudPlatform/kubernetes/issues/137, we should not bound the set of "valid" reasons. Clients should be able to provide a reason upon deletion. Additionally, users should expect the set of system-provided reasons to grow over time and should not treat this list as a comprehensive enumeration of reasons.