[css-values] Clarify that || and && are not associative

[Loirooriol]

https://drafts.csswg.org/css-values-4/#component-combinators already says

For reorderable combinators (||, &&), ordering of the grammar does not matter: components in the same grouping may be interleaved in any order.

But it may be useful to clarify that components in the same grouping can't be interleaved with components of another grouping, e.g.

• a || b || c allows a, b, c or combinations of them in any order
• [ a || b ] || c is like a || b || c but it doesn't allow a c b nor b c a
• a || [ b || c ] is like a || b || c but it doesn't allow b a c nor c a b

Similarly,

• a && b && c allows the permutations of a, b and c
• [ a && b ] && c is like a && b && c but it doesn't allow a c b nor b c a
• a && [ b && c ] is like a && b && c but it doesn't allow b a c nor c a b

In #7884 there is some confusion about this.

[tabatkins]

Added a note with a small example.