Add JSX types used by string builder impl.

Author: sdegutis

site/public/guides/enabling-jsx.md

@@ -115,6 +115,22 @@ declare namespace JSX {
 }
 ```
 
+If you're using `immaculata/jsx-strings.js` then you should use this:
+
+```ts
+declare namespace JSX {
+
+  type ElementChildrenAttribute = { children: string | false | null | undefined | ElementChildrenAttribute['children'][] }
+
+  type Element = string
+
+  type ElementType =
+    | string
+    | ((data: any) => ElementChildrenAttribute['children'])
+
+}
+```
+
 For HTML tag autocompletion and good-enough attr validation:
 
 ```ts