[Calendar] Document usage scope of format tokens

[dreaming-augustin]

Improve the documentation for the original usage, emphasizing that a JavaScript Date object was being returned by the relevant Calendar Behaviors.

Also, document the new Behaviors with the optional format parameter.

Documents: fomantic/Fomantic-UI#3141

[dreaming-augustin]

I was working on using Fomantic Calendar for a project, but I was originally confused by the existing documentation.
I originally thought that the formatter applied to the date we got from $('.ui.calendar').calendar('get date');. It was not clear that a JavaScript Date object was being returned, and that the formatter applied to the cells only.

Now, we get the best of both worlds and the documentation is more explicit.

[lubber-de]

This is wrong. The format token is not only applied to the calendar cells but for several date outputs mentioned as tokens in the settings