docs: Add TypeDef for MessageActivity (#2321)

src/structures/Presence.js

@@ -1,5 +1,12 @@
 const { ActivityTypes, ActivityFlags } = require('../util/Constants');
 
+/**
+ * Activity sent in a message.
+ * @typedef {Object} MessageActivity
+ * @property {string} [partyID] Id of the party represented in activity
+ * @property {number} [type] Type of activity sent
+ */
+
 /**
  * Represents a user's presence.
  */