fix vertical input button group margin (#24498)

* add margin-bottom to .btn of a vertical button group

* match margins of radio/checkbox buttons in a btn-group to the actual buttons

* add comment

scss/_button-group.scss

@@ -197,6 +197,8 @@
 [data-toggle="buttons"] {
   > .btn,
   > .btn-group > .btn {
+    margin-bottom: 0; // Override default `<label>` value
+
     input[type="radio"],
     input[type="checkbox"] {
       position: absolute;