Docs: Clarify return results for a non-existing ID in metadata functions.

[48658] documented that various metadata functions return false for an invalid ID. However, that does not clarify what an invalid ID is: a non-numeric, zero, or negative value. This change adds the clarification in all relevant metadata function docblocks.

Props icopydoc, SergeyBiryukov, davidkryzaniak, audrasjb.
Fixes #51797.
Built from https://develop.svn.wordpress.org/trunk@50641


git-svn-id: http://core.svn.wordpress.org/trunk@50253 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/user.php

@@ -1008,10 +1008,12 @@ function delete_user_meta( $user_id, $meta_key, $meta_value = '' ) {
  * @param string $key     Optional. The meta key to retrieve. By default,
  *                        returns data for all keys.
  * @param bool   $single  Optional. Whether to return a single value.
- *                        This parameter has no effect if $key is not specified.
+ *                        This parameter has no effect if `$key` is not specified.
  *                        Default false.
- * @return mixed An array if $single is false. The value of meta data field
- *               if $single is true. False for an invalid $user_id.
+ * @return mixed An array of values if `$single` is false.
+ *               The value of meta data field if `$single` is true.
+ *               False for an invalid `$user_id` (non-numeric, zero, or negative value).
+ *               An empty string if a valid but non-existing user ID is passed.
  */
 function get_user_meta( $user_id, $key = '', $single = false ) {
 	return get_metadata( 'user', $user_id, $key, $single );