change note in .resizable() mixin for accuracy

Created by: cvrebert

It's 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 648c4689.

/to @mdo for review

Part of the bs-css-hacks documentation initiative!