Remove reference to disallowing :not() within :not() (mdn#36460)

This is no longer the case - The [current spec](https://drafts.csswg.org/selectors/#negation) does not mention any restriction like this.

files/en-us/web/css/_colon_not/index.md

@@ -15,7 +15,7 @@ The `:not()` pseudo-class has a number of [quirks, tricks, and unexpected result
 
 ## Syntax
 
-The `:not()` pseudo-class requires a [selector list](/en-US/docs/Web/CSS/CSS_selectors/Selector_structure#selector_list), a comma-separated list of one or more selectors, as its argument. The list must not contain another negation selector or a [pseudo-element](/en-US/docs/Web/CSS/Pseudo-elements), but any other simple, compound, and complex selectors are allowed.
+The `:not()` pseudo-class requires a [selector list](/en-US/docs/Web/CSS/CSS_selectors/Selector_structure#selector_list), a comma-separated list of one or more selectors, as its argument. The list must not contain a [pseudo-element](/en-US/docs/Web/CSS/Pseudo-elements), but any other simple, compound, and complex selectors are allowed.
 
 ```css-nolint
 :not(<complex-selector-list>) {