DOC Add link to User Guide from Pipeline section (scikit-learn#16829)

Author: justmarkham

doc/modules/classes.rst

@@ -1346,6 +1346,9 @@ Model validation
    :no-members:
    :no-inherited-members:
 
+**User guide:** See the :ref:`combining_estimators` section for further
+details.
+
 .. currentmodule:: sklearn
 
 .. autosummary::