Update backward extremity docs to make it clear that it does not indicate whether we have fetched an events' `prev_events` (#11469)

Spawning from https://github.com/matrix-org/synapse/pull/9445#discussion_r758958181



Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

Add developer documentation to explain room DAG concepts like `outliers` and `state_groups` (#10464)