docs(core): document di and minor renames (#20855)

PR Close #20855

packages/core/src/render3/util.ts

@@ -20,4 +20,14 @@ export function stringify(value: any): string {
   if (typeof value == 'string') return value;
   if (value == null) return '';
   return '' + value;
-}
+}
+
+/**
+ *  Function that throws a "not implemented" error so it's clear certain
+ *  behaviors/methods aren't yet ready.
+ *
+ * @returns Not implemented error
+ */
+export function notImplemented(): Error {
+  return new Error('NotImplemented');
+}