Fixes #10076: Document hide text

Fixes #10076.
This adds a missing helper class and mixin to the **Helper classes** section of the CSS page in our docs. The kicker is that the class and mixin are not the same name, unlike our other classes and mixins in the section (e.g., `.pull-right` is first a class but also mixin-able). This is confusing and super lame, so I've opted to deprecate the mixin and push folks to use the single class name.

```css
// CSS image replacement
// Source: https://github.com/h5bp/html5-boilerplate/commit/aa0396eae757
.hide-text(),
.text-hide() {
  font: ~"0/0" a;
  color: transparent;
  text-shadow: none;
  background-color: transparent;
  border: 0;
}

// Applying the mixin
.text-hide {
  .text-hide();
}
```

That look about right @cvrebert?