Add documentation

Since it is not very clear how isEmpty() works, I added a short note to clarify that it returns true if one of the supplied variables is empty (instead of all of them).

src/functions.php

@@ -133,6 +133,13 @@ function getDirectory(string $path, int $position): ?string
     return $dirs[$position + 1] ?? null;
 }
 
+/**
+ * Determine whether at least one of the supplied variables is empty.
+ *
+ * @param mixed ...$values The values to check.
+ *
+ * @return boolean
+ */
 function isEmpty(mixed ...$values): bool
 {
     $skipValues = array(