Clarify documentation of SkinVector::__construct()

With I5772eb7 merged in core, it's no longer accurate to say that the
$options param is optional.

It's important to note this now, before someone report it as a bug.

Since the method is a public API and technically does not require a
parameter, no one will expect that following the rules (providing no
argument, as in `$skin = new SkinVector`) will throw fatal exception.

Bug: T262233
Change-Id: I771e5cc8ff205943016a26854da18088817b9238

includes/SkinVector.php

@@ -79,6 +79,11 @@ class SkinVector extends SkinMustache {
 	 * in legacy mode.
 	 *
 	 * @inheritDoc
+	 * @param array|null $options Note; this param is only optional for internal purpose.
+	 * 		Do not instantiate Vector, use SkinFactory to create the object instead.
+	 * 		If you absolutely must to, this paramater is required; you have to provide the
+	 * 		skinname with the `name` key. That's do it with `new SkinVector( ['name' => 'vector'] )`.
+	 * 		Failure to do that, will lead to fatal exception.
 	 */
 	public function __construct( $options = [] ) {
 		if ( $this->isLegacy() ) {