[css-masking-1] Fix value definition of deprecated clip property

[cdoublev]

clip is defined in Masking 1 with rect() | auto. rect() is anchored to its definition below the clip property definition table: rect() = rect( <top>, <right>, <bottom>, <left> ). Lastly, clip is deprecated, it is the only property that uses <rect()> with this definition, and there is a new definition of <rect()> in Shapes 1:

rect() = rect( [ <length-percentage> | auto ]{4} [ round <'border-radius'> ]? )

Note: This syntax is similar, but not quite identical, to the legacy rect() function used solely by the clip property.

I believe that rect() in the clip property is ambiguous because it can be interpreted as a function component value named rect with no argument. A terminal or non-terminal must be defined between angled brackets, per the CSS value definition syntax.

To avoid declaring different value definitions for the same type, clip could be defined as rect( <top>, <right>, <bottom>, <left> ) | auto and the definition of <rect()> below the clip definition table could be removed or rewrited in a way that prevent @webref/css to extract it.

[tabatkins]

Correct, the 'clip' grammar is wrong and needs to be replaced with a nonterminal (presumably a specialized one for this property specifically).