change note in .resizable() mixin for accuracy

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 648c4689273647c321dd6e3979d910282e9a9339.

/to @mdo for review

[Part of the bs-css-hacks documentation initiative!](https://github.com/cvrebert/bs-css-hacks/blob/ccd9daad71efbd2c7711795c940bfe14eb51c07b/README.md#resize-requires-overflow-other-than-visible)