[css-filters] Update grammars for CSS filter functions to include value ranges

[weinig]

The CSS filter functions currently specify their value ranges using prose and should be updated to use the CSS bracketed range notation.

For example, blur(), has the grammar:

blur() = blur( <length>? )

with prose following it stating:

"Negative values are not allowed."

Instead, the prose can be removed, and the grammar updated to:

blur() = blur( <length [0,∞]>? )

(the sentence, "The parameter is specified a CSS length, but does not accept percentage values" can also be removed, as percentages are not valid for <length>).