Fix incorrect return type description for `Set#contains()`.

[ZebulanStanphill]

The current return type description is confusing, since it implies that an empty $values would cause the method to return false, when in fact, it would return true.

The corrected description is based on https://www.php.net/manual/en/ds-set.contains.php#refsect1-ds-set.contains-returnvalues