[fill-stroke] Missing quotes around property ref, and not a real "value"

[tidoust]

According to the Value Definition Syntax, for "types that that have the same range of values as a property bearing the same name", "the type name is the property name (complete with quotes)".

These quotes are missing around background in the definition of the fill property.

Also, the "Value" header references the Value Definition Syntax, but the actual value is half-prose: <background> with modifications. This is valid per the syntax, but obviously does not describe the actual content that the property can have. Distinguishing between the actual syntax and the prose would allow to extract the syntax automatically, e.g. to automate some tests or create auto-completion tools for CSS properties.