Improve Self-Managed ConnectionService API docs.

Provide a basic outline of what an app needs to do in order to implement
a self-managed ConnectionService in the class API docs.

Bug: 233932114
Fixes: 235651290
Test: Viewed generated javadoc.
Change-Id: Ibc42d6b75e843c6eb5385bb318d45af58cf8d08a
1 file changed