clearly show that 3 legged oauth is not required (bug 866176)

* add in a clear way for a user landing on the API page to distinguish between 2 and 3 legged oauth
* give a clear example that you can copy and paste right away
I could do that hackery.

The nice think about add_error is that it adds the error directly on to the field, as opposed to on the non-field errors, the UI is cleaner, more accurate and correctly translated.