Update definition of CSS pseudo-classes (#42295)

* Update definition of CSS pseudo-classes

Clarified the definition of CSS pseudo-classes to include selection based on external information.

* Update files/en-us/web/css/reference/selectors/pseudo-classes/index.md

Co-authored-by: Chris Mills <[email protected]>

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Co-authored-by: Chris Mills <[email protected]>

Author: estelle

files/en-us/web/css/reference/selectors/pseudo-classes/index.md

@@ -10,7 +10,7 @@ spec-urls:
 sidebar: cssref
 ---
 
-A [CSS](/en-US/docs/Web/CSS) **_pseudo-class_** is a keyword added to a selector that lets you style a specific state of the selected element(s). For example, the pseudo-class {{CSSxRef(":hover")}} can be used to select a button when a user's pointer hovers over the button and this selected button can then be styled.
+A [CSS](/en-US/docs/Web/CSS) **_pseudo-class_** is a keyword added to a selector that lets you select elements based on information that lies outside of the document tree, such as a specific state of the selected element(s). For example, the pseudo-class {{CSSxRef(":hover")}} can be used to style a button when a user's pointer hovers over it.
 
 ```css
 /* Any button over which the user's pointer is hovering */