Document that the JavaScript singleton may be disabled at build-time

(cherry picked from commit 0422027f07efb8e7e27e02ebec82319fdab68e52)

doc/classes/JavaScript.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		The JavaScript singleton is implemented only in the HTML5 export. It's used to access the browser's JavaScript context. This allows interaction with embedding pages or calling third-party JavaScript APIs.
+		[b]Note:[/b] This singleton can be disabled at build-time to improve security. By default, the JavaScript singleton is enabled. Official export templates also have the JavaScript singleton enabled. See [url=https://docs.godotengine.org/en/latest/development/compiling/compiling_for_web.html]Compiling for the Web[/url] in the documentation for more information.
 	</description>
 	<tutorials>
 		<link>https://docs.godotengine.org/en/latest/getting_started/workflow/export/exporting_for_web.html#calling-javascript-from-script</link>