Update and enhance the docs for retrieve_widgets().

Props zieladam, hellofromtonya.
Fixes #53811.

git-svn-id: https://develop.svn.wordpress.org/trunk@51842 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/widgets.php

@@ -1255,7 +1255,15 @@ function _wp_sidebars_changed() {
 }
 
 /**
- * Look for "lost" widgets, this has to run at least on each theme change.
+ * Validates and remaps any "orphaned" widgets to wp_inactive_sidgets sidebar, and saves the widget settings.
+ * This has to run at least on each theme change.
+ *
+ * For example, let's say theme A has a "footer" sidebar, and theme B doesn't have one.
+ * After switching from theme A to theme B, all the widgets previously assigned to the footer would be inaccessible.
+ * This function detects this scenario, and moves all the widgets previously assigned to the footer under wp_inactive_widgets.
+ *
+ * Despite the word "retrieve" in the name, this function actually updates the database and the global $sidebars_widgets.
+ * For that reason it should not be run from the front end unless the param value is 'customize' (to bypass the database write).
  *
  * @since 2.8.0
  *
@@ -1310,6 +1318,7 @@ function retrieve_widgets( $theme_changed = false ) {
 	$sidebars_widgets['wp_inactive_widgets'] = array_merge( $lost_widgets, (array) $sidebars_widgets['wp_inactive_widgets'] );
 
 	if ( 'customize' !== $theme_changed ) {
+		// Update the widgets settings in the database.
 		wp_set_sidebars_widgets( $sidebars_widgets );
 	}