doc: include null as a return value for require.resolve.paths()

Update documentation of modules.md for require.resolve.paths()
to include the possibillity of null as a return value if
the request string given matches a core module name.

Fixes: nodejs#18230

doc/api/modules.md

@@ -626,9 +626,11 @@ added: v8.9.0
 -->
 
 * `request` {string} The module path whose lookup paths are being retrieved.
-* Returns: {Array}
+* Returns: {Array|null}
 
-Returns an array containing the paths searched during resolution of `request`.
+Returns an array containing the paths searched during resolution of `request` or
+null if the `request` string references a core module, for example `http` or
+`fs`.
 
 ## The `module` Object
 <!-- YAML