[css-writing-modes] caption-side logical values should only be in css-logical-props?

Writing Modes spec defining logical values only for this property does not
look consistent.

It's already defined in CSS Logical Properties Level 1[1], so I think we
should define only interpretation of existing values in Writing Modes spec,
and delegate all logical values to CSS Logical Properties spec.

Does this look reasonable?

[1] https://drafts.csswg.org/css-logical-props/

/koji