Add some clarifying comments

res/css/_common.scss

@@ -110,6 +110,10 @@ textarea {
     color: $primary-fg-color;
 }
 
+// This is used to hide the standard outline added by browsers for
+// accessible (focusable) components. Not intended for buttons, but
+// should be used on things like focusable containers where the outline
+// is usually not helping anyone.
 .mx_HiddenFocusable {
     outline: none;
 }