Add docs for notify and query subscription pattern

Useful for comment threads / chat threads where the ideal pattern is to use subscriptions for "notifications" followed by a re-query instead of a live query for the entire thread.

No file, test, or documentation entry point is named. Start by locating the existing GraphQL subscription documentation and checking how notification subscriptions and follow-up re-queries are currently described. Done means the comment-thread and chat-thread pattern is documented clearly, including when to re-query instead of maintaining a live query.

Written by the indexing model from the issue text.