[css3-ui] Clearly define when :read-only and :read-write match

Currently the spec is pretty vague about when an element is read-only and
when it is writable. It should define clear rules for that.

E.g. it should mention that setting the 'contenteditable' HTML attribute
makes an element writable, and which elements by default count as writable
like text areas and text inputs. Also it should clarify whether disabled
inputs are matched as read-only and mention the interaction between the
'readonly' HTML attribute.

Sebastian