[selectors-4] Clarify the value range of pseudo selectors

[cdoublev]

§ Grammar defines pseudos with a relatively open range of values that does not come with any restriction, at least not within reasonable reading proximity.

§ Pseudo-elements says that pseudos are defined by CSS specifications and the host language.

§ Invalid Selectors says that UAs must treat as invalid any pseudos for which they have no usable level of support. But in a non-normative note, as reported in #8305. § Appendix B: [...] Web Compat adds an exception: UAs must support ::webkit-unknown even if they have no support for it.

A note similar to the one below could be added in § Grammar:

The <pseudo-element-selector> and <pseudo-class-selector> productions are intentionally open to represent any pseudo-class and pseudo-element defined in CSS specifications or by the host language, that is supported by the UA. As per Patial implementations, other pseudos are otherwise invalid, unless otherwise specified (like in § Appendix B: [...] Web Compat).