Document working with connections directly.

[tomchristie]

We expose the following through our public API...

• HTTPConnection / AsyncHTTPConnection
• HTTP11Connection / AsyncHTTP11Connection
• HTTP2Connection / AsyncHTTP2Connection

However we don't have them publicly documented.

Users wouldn't typically need to work directly with connections since you normally want to be sending requests at the level of the connection pool, but it'd be really valuable for us to document this layer of the API.

With a similar motivation to our docs & API work on #699, helping expose the API layering in httpcore is valuable because it demonstrates how the various parts fit together & allows users to isolate issues more clearly.