Docs: Add a note to the DocBlock for `wp_signon()` mentioning that it doesn't handle setting the current user.

Also outlines the conditions that must be met for is_user_logged_in() to work with it.

Part props SergeyBiryukov.
Fixes #28116.

Built from https://develop.svn.wordpress.org/trunk@40943


git-svn-id: http://core.svn.wordpress.org/trunk@40793 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/user.php

@@ -17,6 +17,11 @@
  * set for a longer period depending on if the 'remember' credential is set to
  * true.
  *
+ * Note: wp_signon() doesn't handle setting the current user. This means that if the
+ * function is called before the {@see 'init'} hook is fired, is_user_logged_in() will
+ * evaluate as false until that point. If is_user_logged_in() is needed in conjunction
+ * with wp_signon(), wp_set_current_user() should be called explicitly.
+ *
  * @since 2.5.0
  *
  * @global string $auth_secure_cookie

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-alpha-40942';
+$wp_version = '4.9-alpha-40943';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.