Better document the primary use cases for the sink feature

[kgriffs]

Improve the docstring and/or add some recipes showing how to implement common use cases via Falcon's sink feature.

Use cases include:

• [ ] Creating a smart proxy (could be an API gateway, or something that optimizes or redacts responses from backend, etc.)
• [ ] Migrating to a new API iteratively. Falcon handles the parts of the API that have been implemented so far, while using a sink to proxy unimplemented endpoints to the legacy backend.
• [ ] Handling complicated routes that aren't (yet) supported by the default router