Plugins: Clarify documentation for when current_action and current_filter can return false.

If `current_action` or `current_filter` is called outside of an action/filter, then these functions will return false.

Props debarghyabanerjee, mukesh27, peterwilsoncc, SirLouen.
Fixes #63479.


git-svn-id: https://develop.svn.wordpress.org/trunk@60258 602fd350-edb4-49c9-b593-d223f7449a82

Author: aaronjorbin

src/wp-includes/plugin.php

@@ -359,7 +359,7 @@ function remove_all_filters( $hook_name, $priority = false ) {
  *
  * @global string[] $wp_current_filter Stores the list of current filters with the current one last
  *
- * @return string Hook name of the current filter.
+ * @return string|false Hook name of the current filter, false if no filter is running.
  */
 function current_filter() {
 	global $wp_current_filter;
@@ -632,7 +632,7 @@ function remove_all_actions( $hook_name, $priority = false ) {
  *
  * @since 3.9.0
  *
- * @return string Hook name of the current action.
+ * @return string|false Hook name of the current action, false if no action is running.
  */
 function current_action() {
 	return current_filter();