Docs: Add file header DocBlock for `wp-signup.php`.

Follow-up to [https://mu.trac.wordpress.org/changeset/543 mu:543], [https://mu.trac.wordpress.org/changeset/557 mu:557], [12603].

Props jdahir0789, apermo, aboelfotoh.
Fixes #62254.
Built from https://develop.svn.wordpress.org/trunk@59378


git-svn-id: http://core.svn.wordpress.org/trunk@58764 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.8-alpha-59377';
+$wp_version = '6.8-alpha-59378';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.

wp-signup.php

@@ -1,4 +1,11 @@
 <?php
+/**
+ * WordPress Signup Page
+ *
+ * Handles the user registration and site creation process for multisite installations.
+ *
+ * @package WordPress
+ */
 
 /** Sets up the WordPress Environment. */
 require __DIR__ . '/wp-load.php';