Adds x-data to x-mask documentation

[tjbarber]

Spent more time than I'd like to admit trying to add x-mask to a field just to have it do nothing. It wasn't until I found this discussion that I realized that I was missing x-data which wasn't mentioned in the docs for this plugin.

I think it makes sense to highlight to the user that this is required, either with something like this or an alert near the top of the page.

[ekwoka]

The way this is done introduces the possibility of the opposite mis understanding now, where the reader thinks that x-data is always required.

Perhaps following how this pr #4307 handles it is acceptable?

[tjbarber]

I would say that would be acceptable if your alert was also on x-mask documentation!