Values of CSS properties

I see this pattern in many CSS properties. For example, http://docs.webplatform.org/wiki/css/properties/background-position
Some have generic data types as values (e.g. length), others try to use specific examples.
What’s the correct way?
Personally, I think specific examples can be confusing, since the reader needs to extrapolate the general rule.

Lea Verou
W3C developer relations
http://w3.org/people/all#lea ✿ http://lea.verou.me ✿ @leaverou