Clarify comma-separate values in value of the scope

[dontcallmedom]

As reported in w3c/reffy#229 (comment), the scope of a definition (as set through data-dfn-for) is set to be a comma-separate value, some of these scopes contains commas themselves.

This could be either clarified to take into account the specifics of comma-inside-parenthesis (as bikeshed and reffy currently do), or be fixed with some sort of escape mechanism.