change note in .resizable() mixin for accuracy

Not a Safari bug. It's standards-compliant.

Quoting from http://www.w3.org/TR/css3-ui/#resize :
> ### 8.1. `resize` property
>
> * Applies to: elements with `overflow` other than `visible`
>
> The `resize` property applies to elements whose computed `overflow` value is something other than `visible`.

Original comment was added in commit 648c468927.

less/mixins/resize.less

@@ -2,5 +2,5 @@
 
 .resizable(@direction) {
   resize: @direction; // Options: horizontal, vertical, both
-  overflow: auto; // Safari fix
+  overflow: auto; // Per CSS3 UI, `resize` only applies when `overflow` isn't `visible`
 }