chore: corrected the docs the getRelationsForEvent function

src/models/event-timeline-set.js

@@ -695,10 +695,11 @@ EventTimelineSet.prototype.compareEventOrdering = function(eventId1, eventId2) {
  * The type of relation involved, such as "m.annotation", "m.reference", "m.replace", etc.
  * @param {String} eventType
  * The relation event's type, such as "m.reaction", etc.
+ * @throws If <code>eventId, relationType or eventType</code> are not valid.
  *
  * @returns {?Relations}
- * A container for relation events, or undefined if either an event argument is invalid or
- * aggregation is disabled.
+ * A container for relation events or undefined if there are no relation events for
+ * the relationType.
  */
 EventTimelineSet.prototype.getRelationsForEvent = function(
     eventId, relationType, eventType,