Correct comment to reflect reality. b=317480

layout/xul/base/src/grid/nsGridRowGroupFrame.h

@@ -49,9 +49,10 @@
 #include "nsBoxFrame.h"
 
 /**
- * A frame representing a grid row (or column) group, which is an
- * element that is a child of a grid and contains all the rows (or
- * all the columns).
+ * A frame representing a grid row (or column) group, which is usually
+ * an element that is a child of a grid and contains all the rows (or
+ * all the columns).  However, multiple level of groups are allowed, so
+ * the parent or child could instead be another group.
  */
 class nsGridRowGroupFrame : public nsBoxFrame
 {