[css-typed-om] introduction to CSSNumericValue should be clearer that list is interfaces inheriting from it

[dbaron]

The opening of the section on CSSNumericValue says:

CSSNumericValue objects represent CSS values that are numeric in nature (<number>s, <percentage>s, <dimension>s).

• CSSUnitValue objects represent values that contain a single unit type (for example "42px").

• CSSMathValue objects represent math expressions, which can contain more than one value/unit (for example "calc(56em + 10%)").

This reads in a slightly confusing way, until I realized that the bulleted list was two interfaces that inherit from CSSNumericValue. It would probably be a good bit clearer why this is structured the way it is (with three parallel sentences, but only the latter two in a list), by introducing the list with a sentence, such as "Two interfaces inherit from CSSNumericValue:"