.form-control: add note about <select> & border-radius

Refs #15340

less/forms.less

@@ -123,7 +123,7 @@ output {
   background-color: @input-bg;
   background-image: none; // Reset unusual Firefox-on-Android default style; see https://github.com/necolas/normalize.css/issues/214
   border: 1px solid @input-border;
-  border-radius: @input-border-radius;
+  border-radius: @input-border-radius; // Note: This has no effect on <select>s in some browsers, due to the limited stylability of <select>s in CSS.
   .box-shadow(inset 0 1px 1px rgba(0,0,0,.075));
   .transition(~"border-color ease-in-out .15s, box-shadow ease-in-out .15s");