DOC: Add warning for newbs not to edit auto-generated file
@jreback, here is the proposed changed we talked about in [this PR](https://github.com/pydata/pandas/pull/10337#issuecomment-111822584) for other fellow newbs that make the same mistake I did.

You said "and more importantly prob a note in `internals.rst` about how/what to change", but I didn't understand that.  `internals.rst` is not auto-generated, right?  So, why would I add a note there?  Perhaps you wanted a note to appear in `internals.html`?  That seems unnecessary since if the user has that file on their machine, they must have intentionally generated it...
