add missing docstring Returns section

lib/old_sets.bzl

diff --git a/lib/old_sets.bzl b/lib/old_sets.bzl
index f62e03d..8cb6862 100644
--- a/lib/old_sets.bzl
+++ b/lib/old_sets.bzl
@@ -51,6 +51,8 @@
 
     Args:
       val: The value to be optionally converted and returned.
+    Returns:
+      The converted value.
     """
     if type(val) == _depset_type:
         return val.to_list()