Clarify that we mark as outliers because we don't have any state for them (#12345)

As discussed at https://github.com/matrix-org/synapse/pull/12179#discussion_r837263852

changelog.d/12345.doc

@@ -0,0 +1 @@
+Updates to the Room DAG concepts development document to clarify that we mark events as outliers because we don't have any state for them.

docs/development/room-dag-concepts.md

@@ -39,7 +39,8 @@ yet correlated to the DAG.
 Outliers typically arise when we fetch the auth chain or state for a given
 event. When that happens, we just grab the events in the state/auth chain,
 without calculating the state at those events, or backfilling their
-`prev_events`.
+`prev_events`. Since we don't have the state at any events fetched in that
+way, we mark them as outliers.
 
 So, typically, we won't have the `prev_events` of an `outlier` in the database,
 (though it's entirely possible that we *might* have them for some other