add reference to PEP 525 for async generator semantics

Author: adqm

peps/pep-0798.rst

@@ -217,7 +217,9 @@ generator::
         for x in it:
             yield from expr
 
-Since ``yield from`` is not allowed inside of async generators, the equivalent
+Since ``yield from`` is not allowed inside of async generators (see the section
+of :pep:`525` on `Asynchronous yield from
+<https://peps.python.org/pep-0525/#asynchronous-yield-from>`__), the equivalent
 for ``(*expr async for x in ait())``, is more like the following (though of
 course this new form should not define or reference the looping variable
 ``i``)::