docs: add missing method mentions in oauth1.rfc5849.request_validator.RequestValidator docstring
docs: add missing method usage info in oauth1 RequestValidator.get_realms() docstring
docs: add more verbose docstring in oauth1 RequestValidator methods

- add link to Section 2.3 RFC 5849 that explains why there is a need for
  token invalidation and give a clue how it can be invalidated (refs #202)
- add example implementation of validate_realms
- explain why `verify_request_token` does not double functionality of
  `validate_request_token` (refs #185)
Merge pull request #250 from swistakm/fix/oauth1-request-validator-docs-fixes

OAuth1.0 RequestValidator multiple docs fixes