Docs: Mention the globals key in the no-undef docs (#9867)

* Mention the `globals` key

* Docs: mention specifying globals

* Docs: mention the globals key in the no-undef Rule Details section

docs/rules/no-undef.md

@@ -4,7 +4,7 @@ This rule can help you locate potential ReferenceErrors resulting from misspelli
 
 ## Rule Details
 
-Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a `/*global ...*/` comment.
+Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a `/*global ...*/` comment, or specified in the [`globals` key in the configuration file](https://eslint.org/docs/user-guide/configuring#specifying-globals). A common use case for these is if you intentionally use globals that are defined elsewhere (e.g. in a script sourced from HTML).
 
 Examples of **incorrect** code for this rule: