add jsdocs

src/base-apis.js

@@ -438,7 +438,16 @@ MatrixBaseApis.prototype.createRoom = function(options, callback) {
         callback, "POST", "/createRoom", undefined, options,
     );
 };
-
+/**
+ * Fetches relations for a given event
+ * @param {string} roomId the room of the event
+ * @param {string} eventId the id of the event
+ * @param {string} relationType the rel_type of the relations requested
+ * @param {string} eventType the event type of the relations requested
+ * @param {Object} opts options with optional values for the request.
+ * @param {Object} opts.from the pagination token returned from a previous request as `next_batch` to return following relations.
+ * @return {Object} the response, with chunk and next_batch.
+ */
 MatrixBaseApis.prototype.fetchRelations =
     async function(roomId, eventId, relationType, eventType, opts) {
     const queryParams = {};