Docs: Document $wp_version global in wp_initial_constants().

Props mt8.biz.
Fixes #42259.

git-svn-id: https://develop.svn.wordpress.org/trunk@41959 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/default-constants.php

@@ -12,7 +12,8 @@
  *
  * @since 3.0.0
  *
- * @global int $blog_id
+ * @global int    $blog_id    The current site ID.
+ * @global string $wp_version The WordPress version string.
  */
 function wp_initial_constants() {
 	global $blog_id;