Add a comment regarding the negative bottom margin on the dialog actions.

Author: crisbeto

src/lib/dialog/dialog.scss

@@ -46,6 +46,9 @@ md-dialog-actions, [md-dialog-actions], mat-dialog-actions, [mat-dialog-actions]
   display: block;
 
   &:last-child {
+    // If the actions are the last element in a dialog, we need to pull them down
+    // over the dialog padding, in order to avoid the action's padding stacking
+    // with the dialog's.
     margin-bottom: -$md-dialog-padding;
   }
 }