Add information on MSC3966: event_property_contains.

content/client-server-api/modules/push.md

@@ -337,6 +337,50 @@ The following `m.federate` values will NOT match:
  * `1` (do not cast types)
 {{% /boxes/note %}}
 
+**`event_property_contains`**
+
+This matches if an array property of an event exactly contains a value. Parameters:
+
+-   `key`: The dot-separated path of the property of the event to match, e.g.
+    `content.body`.
+
+-   `value`: The value to match against.
+
+The array values are matched exactly and only supports strings, integers, booleans,
+and `null`. Array values not of those types are ignored.
+
+If the property specified by `key` is completely absent from the event, or is not
+an array, then the condition will not match.
+
+{{% boxes/note %}}
+For example, if `key` is `content.alt_aliases`, and `value` is `"#myroom:example.com"`,
+then the following event will match:
+
+```json
+{
+  "content": {
+    "alias": "#somewhere:localhost",
+    "alt_aliases": [
+      "#somewhere:example.org",
+      "#myroom:example.com"
+    ]
+  },
+  "event_id": "$143273582443PhrSn:example.org",
+  "origin_server_ts": 1432735824653,
+  "room_id": "!jEsUZKDJdhlrceRyVU:example.org",
+  "sender": "@example:example.org",
+  "state_key": "",
+  "type": "m.room.canonical_alias",
+  "unsigned": {
+    "age": 1234
+  }
+}
+```
+
+The following `alt_aliases` values will NOT match:
+ * `":example.com"` (partial values do not match)
+{{% /boxes/note %}}
+
 **`contains_display_name`**
 
 This matches messages where `content.body` contains the owner's display name in