Add link to PEP 525 in Expressions. (GH-10333)

(cherry picked from commit bfe1839aa994f0d84471254418a4ecfa7c7c9b9c)

Co-authored-by: Andrés Delfino <adelfino@gmail.com>
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
index de9ff57..3b50efe 100644
--- a/Doc/reference/expressions.rst
+++ b/Doc/reference/expressions.rst
@@ -421,7 +421,7 @@
     def gen():  # defines a generator function
         yield 123
 
-    async def agen(): # defines an asynchronous generator function (PEP 525)
+    async def agen(): # defines an asynchronous generator function
         yield 123
 
 Due to their side effects on the containing scope, ``yield`` expressions
@@ -506,6 +506,10 @@
       The proposal to introduce the :token:`yield_from` syntax, making delegation
       to sub-generators easy.
 
+   :pep:`525` - Asynchronous Generators
+      The proposal that expanded on :pep:`492` by adding generator capabilities to
+      coroutine functions.
+
 .. index:: object: generator
 .. _generator-methods: