Comment tweak based on CR feedback.

Author: Michael Lehenbauer

firebase-firestore/src/main/java/com/google/firebase/firestore/core/Query.java

@@ -82,7 +82,10 @@ public Query(
     this.endAt = endAt;
   }
 
-  /** Initializes a Query with a path and (optional) collectionGroup. */
+  /**
+   * Initializes a Query with a path and (optional) collectionGroup. Path must currently be
+   * empty in the case of a collection group query.
+   */
   public Query(ResourcePath path, @Nullable String collectionGroup) {
     this(
         path,