Docs: Add missing documentation for fallback_gap_value param

Follow-up for [53420]. 

Props SergeyBiryukov.
See #55567.



git-svn-id: https://develop.svn.wordpress.org/trunk@53421 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/block-supports/layout.php

@@ -41,6 +41,7 @@ function wp_register_layout_support( $block_type ) {
  * @param boolean $has_block_gap_support         Whether the theme has support for the block gap.
  * @param string  $gap_value                     The block gap value to apply.
  * @param boolean $should_skip_gap_serialization Whether to skip applying the user-defined value set in the editor.
+ * @param string  $fallback_gap_value            The custom fallback value for block gap.
  * @return string CSS style.
  */
 function wp_get_layout_style( $selector, $layout, $has_block_gap_support = false, $gap_value = null, $should_skip_gap_serialization = false, $fallback_gap_value = '0.5em' ) {