Add a ref.

Author: ericsnowcurrently

Doc/howto/multiple-interpreters.rst

@@ -223,7 +223,8 @@ You can just as easily call a function in another interpreter::
 
 In fact, nearly all Python functions and callables are supported,
 with the notable exception of closures.  Support includes arguments
-and return values, which we'll explore soon.
+and return values, which we'll
+:ref:`explore soon <interp-tutorial-return-values>`.
 
 Builtin functions always execute in the target interpreter's
 :mod:`!__main__` module::
@@ -673,6 +674,8 @@ between interpreters.
 
 In this half of the tutorial, we explore various ways you can do so.
 
+.. _interp-tutorial-return-values:
+
 Call Args and Return Values
 ---------------------------