diff --git a/docs/matrix.md b/docs/matrix.md
index b0c0040f..5bc4328e 100644
--- a/docs/matrix.md
+++ b/docs/matrix.md
@@ -67,6 +67,12 @@ services.hyperhive.swarm.matrix = {
 };
 ```
 
+A rebuild on a host that already has a homeserver prints a
+`hive-matrix: WARNING — … serverName is unset` line when this is missing,
+naming the value it is about to default to. That warning is why this
+section exists; it never fails the rebuild, so it is on you to act on it
+before the ids are minted.
+
 ## Default-closed firewall
 
 `openFirewall` defaults to `false` (secure-by-default): the host