Merge pull request #12357 from twbs/border-box-note

add note in Less source about global `box-sizing: border-box;`

less/scaffolding.less

@@ -4,7 +4,10 @@
 
 
 // Reset the box-sizing
-
+//
+// Heads up! This reset may cause conflicts with some third-party widgets.
+// For recommendations on resolving such conflicts, see
+// http://getbootstrap.com/getting-started/#third-box-sizing
 * {
   .box-sizing(border-box);
 }