self is not properly documented

[iamdefinitelyahuman]

The only references to self in the documentation are in the examples, and briefly when explaining how to call a private function. Given this is a fairly fundamental variable when writing Vyper contracts, it deserves some proper documentation.

Related, re: #1696, we should very clearly document that Vyper disallows external calls back into the same contract, the rationale behind this, and (once the issue is closed), how to perform this action via raw_call.