Document how to specify multiple CSS classes for CKEditor's styles dropdown

Problem/Motivation

When creating a wysiwyg link i'd like to add a couple of classes to that link to make it stand out as a button. Drupal 8 has a nice and easy way to create a "Styles dropdown" in the CKEditor Settings for text formats located for example in /admin/config/content/formats/manage/full_html

Screenshot: https://www.evernote.com/shard/s245/sh/ee50272c-4699-4c94-9bcf-301e1f4fd...

You can the following to be able to select a link in the WYSIWYG and say "Hey, this is a button":

a.button--primary|Button Primary

The problem is: You can only add one CSS class to each dropdown element this way it seems. I.e: for a link to be buttonized using the Bartik theme, you'll also need the "button" class to that link.

I'd like to be able to write something like:

a.button--primary button|Button Primary

But then I'll get the following error message on save:

The provided list of styles is syntactically incorrect.

Proposed resolution

A: If I am wrong and this is after all possible the following help text should have an instruction for that: (Suggestions needed!)
"A list of classes that will be provided in the "Styles" dropdown. Enter one class on each line in the format: element.class|Label. Example: h1.title|Title.
These styles should be available in your theme's CSS file."

or B: Change the validation for the CKEditor Dropdown styles list on save so multiple CSS classes is allowed for each Dropdown element.

Remaining tasks

Documentations changes and possibly development for suggestion B above.

User interface changes

changes to user interface text on admin/config/content/formats/manage/full_html

API changes

I don't know.

Data model changes

I don't know.