Clarify how useDataAttrOptions works

Because `useDataAttrOptions` uses jQuery's `data()` method to cache the options on the element (instead of `attr()`), no `data-i18n-options` attribute is ever added.

Also added a few lines to clarify how this feature is meant to be used.
Merge pull request #192 from adamesque/patch-1

Clarify how useDataAttrOptions works