build(aio): add @nodoc alias to the @internal tag (#21024)

The `@internal` tag prevents code items from appearing in the docs and the
typings files. You can now use `@nodoc` if you only want it to be excluded
from the docs and not the typings files.

Closes #20990

PR Close #21024

aio/tools/transforms/angular-api-package/tag-defs/internal.js

@@ -1,5 +1,16 @@
+/**
+ * Use this tag to ensure that dgeni does not include this code item
+ * in the rendered docs.
+ *
+ * The `@internal` tag indicates to the compiler not to include the
+ * item in the public typings file.
+ * Use the `@nodoc` alias if you only want to hide the item from the
+ * docs but not from the typings file.
+ */
 module.exports = function() {
   return {
-    name: 'internal', transforms: function() { return true; }
+    name: 'internal',
+    aliases: ['nodoc'],
+    transforms: function() { return true; }
   };
 };