[selectors-4] Clarify how to parse `S` in `<:nth-child()>`

[cdoublev]

The value definitions of <:nth-child()> and <:nth-last-child()> includes S defined as a <compound-selector-list> parsed as a forgiving selector list. But the first step of parsing as a forgiving selector list is:

1. Parse a list of <complex-selector>s from input, and let selector list be the result.

I assume that :nth-child(odd of ul > li) should be invalid because ul > li is a <complex-selector>, but :nth-child(odd of li.foo, li.bar) is valid because li.foo, li.bar is a <compound-selector-list>.

I guess that this situation comes from the resolution and the commits mentionned in #1292 and #3760.

I suggest to replace S by <forgiving-compound-selector-list> and to define it as <forgiving-compound-selector-list> is identical to <forgiving-selector-list>, except it parses its components as <compound-selector> rather than <complex-selector>. Selectors 5 can replace <forgiving-compound-selector-list> by <forgiving-selector-list> if <complex-selector>s become valid.

[cdoublev]

Actually the resolution is to allow <complex-selector> as S, sorry. I'm closing this duplicate and I assume that S will be replaced by <forgiving-selector-list>.