* Improve/upgrade OAuth2 scopes support with `SecurityScopes`:
* `SecurityScopes` can be declared as a parameter like `Request`, to get the scopes of all super-dependencies/dependants.
* Improve `Security` handling, merging scopes when declaring `SecurityScopes`.
* Allow using `SecurityBase` (like `OAuth2`) classes with `Depends` and still document them. `Security` now is needed only to declare `scopes`.
* Updated docs about: <a href="https://fastapi.tiangolo.com/tutorial/security/oauth2-jwt/" target="_blank">OAuth2 with Password (and hashing), Bearer with JWT tokens</a>.
* New docs about: <a href="https://fastapi.tiangolo.com/tutorial/security/oauth2-scopes/" target="_blank">OAuth2 scopes</a>.
* PR <a href="https://github.com/tiangolo/fastapi/pull/141" target="_blank">#141</a>.