Docs: Reduce confusion in @since MU notationss by indicating the WP version (3.0.0) the code was merged into core while retaining the original context.

Props sathyapulse.
Fixes #41509.


git-svn-id: https://develop.svn.wordpress.org/trunk@41200 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-theme.php

@@ -1270,7 +1270,7 @@ final class WP_Theme implements ArrayAccess {
 		/**
 		 * Filters the array of themes allowed on the network.
 		 *
-		 * @since MU
+		 * @since MU (3.0.0)
 		 *
 		 * @param array $allowed_themes An array of theme stylesheet names.
 		 */