bpo-36404: recommend DeprecationWarning over PendingDeprecationWarning (GH-12505)

diff --git a/Doc/library/exceptions.rst b/Doc/library/exceptions.rst
index 57ed291..2509380 100644
--- a/Doc/library/exceptions.rst
+++ b/Doc/library/exceptions.rst
@@ -694,6 +694,14 @@
    Base class for warnings about features which will be deprecated in the
    future.
 
+   .. note::
+      PendingDeprecationWarning was introduced as an "ignored by default"
+      version of DeprecationWarning.  But :exc:`DeprecationWarning` is also
+      ignored by default since Python 2.7 and 3.2.
+      There is not much difference between PendingDeprecationWarning and
+      DeprecationWarning nowadays.  DeprecationWarning is recommended
+      in general.
+
 
 .. exception:: SyntaxWarning