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

2 files changed, 3 insertions, 1 deletions

diff --git a/changelog.d/12345.doc b/changelog.d/12345.doc
new file mode 100644

index 0000000000..0dcf836e68
--- /dev/null
+++ b/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.
diff --git a/docs/development/room-dag-concepts.md b/docs/development/room-dag-concepts.md

index 3eb4d5acc4..76709487f8 100644
--- a/docs/development/room-dag-concepts.md
+++ b/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