Improve CDK documentation around caching and child streams

### Tell us about the documentation you'd like us to add or update.
[These docs ](https://docs.airbyte.com/connector-development/cdk-python/http-streams#nested-streams-and-caching) don't provide enough guidance on how a user should interact with substreams.

For example:
1. How could a child stream "become incremental" if its own endpoint doesn't support incremental syncs, but the parent supports incremental syncs?
2. What does inheriting from `HttpSubStream` do as opposed to just inheriting `HttpStream`?