Clarify removal of border radii

scss/_button-group.scss

@@ -48,11 +48,12 @@
     @include border-right-radius(0);
   }
 
-  // - Target second buttons which are not part of toggle buttons
-  // - Target third or more child
-  // - Target buttons in a button group
-  > :not(.btn-check) + .btn,
+  // The left radius should be 0 if the button is:
+  // - the "third or more" child
+  // - the second child and the previous element isn't `.btn-check` (making it the first child visually)
+  // - part of a btn-group which isn't the first child
   > .btn:nth-child(n + 3),
+  > :not(.btn-check) + .btn,
   > .btn-group:not(:first-child) > .btn {
     @include border-left-radius(0);
   }