Docs: Document that various metadata functions return false for an invalid ID.

Props salcode.
See #40519.
Built from https://develop.svn.wordpress.org/trunk@48658


git-svn-id: http://core.svn.wordpress.org/trunk@48420 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/taxonomy.php

@@ -1292,7 +1292,7 @@ function delete_term_meta( $term_id, $meta_key, $meta_value = '' ) {
  *                        This parameter has no effect if $key is not specified.
  *                        Default false.
  * @return mixed An array if $single is false. The value of the meta field
- *               if $single is true.
+ *               if $single is true. False for an invalid $term_id.
  */
 function get_term_meta( $term_id, $key = '', $single = false ) {
 	return get_metadata( 'term', $term_id, $key, $single );