DeepCollection()._obj not documented

[BenjaminEchelmeier]

Looking at the README, I wasn't able to find this for a refresh--it's a handy attribute that should probably be clear from the get-go.

[nixjdm]

I can mention it. There should be some qualifications with it's usage, because I'd think most of the time it isn't needed because:

1. the DeepCollection object is a bona fide instance of the same type as _obj, so it should behave the same except where functionality is overridden
2. default usage of a dc object should still match what would have happened with _obj. E.g. you'll still get a KeyError or IndexError if you supply a single key to __getitem__, most of the time.

There is some flexibility in 2 though, so it may be simpler to use _obj if you really want those errors to pop up, and don't want to think about how to achieve that with a dc. That will of course mean you can't supply a path, but that's simpler than getting a [path] to return an error. I realize now that I haven't made strict be a kwarg on DeepCollection, so I'll add that.