doc: add documentation for Security dependencies

[adriangb]

Make sure to include the ability to dynamically configure security dependencies in lifespan events: https://github.com/adriangb/xpresso/blob/88840e95bddb6f8b27dfab83f44485fc1b8f97c9/tests/test_security/test_security_api_key_header_deferred.py#L12-L34