Docs: Expand delete_theme() return value, fix typo in delete_plugins() return value.

See #42505.
Built from https://develop.svn.wordpress.org/trunk@42778


git-svn-id: http://core.svn.wordpress.org/trunk@42608 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/theme.php

@@ -13,9 +13,10 @@
  *
  * @global WP_Filesystem_Base $wp_filesystem WordPress filesystem subclass.
  *
- * @param string $stylesheet Stylesheet of the theme to delete
- * @param string $redirect Redirect to page when complete.
- * @return void|bool|WP_Error When void, echoes content.
+ * @param string $stylesheet Stylesheet of the theme to delete.
+ * @param string $redirect   Redirect to page when complete.
+ * @return bool|null|WP_Error True on success, false if `$stylesheet` is empty, WP_Error on failure.
+ *                            Null if filesystem credentials are required to proceed.
  */
 function delete_theme( $stylesheet, $redirect = '' ) {
 	global $wp_filesystem;