document unstable prefixes/feature flags

proposals/2432-revised-alias-publishing.md

@@ -219,4 +219,11 @@ None currently identified.
 
 ## Unstable prefix
 
-TBD.
+While this feature is in development, the following names will be in use:
+
+| Proposed final name | Name while in development |
+| `GET /_matrix/client/r0/rooms/{roomId}/aliases` | `GET /_matrix/client/unstable/org.matrix.msc2432/rooms/{roomId}/aliases` |
+
+Servers will indicate support for the new endpoint via a non-empty value for feature flag
+`org.matrix.msc2432` in `unstable_features` in the response to `GET
+/_matrix/client/versions`.