Doc: update PendingDeprecationWarning explanation (GH-12837)

Keep the nudge towards DeprecationWarning, but remove the
"Note" markup and generally shorten the description.

Ref: https://github.com/python/cpython/pull/12505/filesGH-r273978757
(cherry picked from commit a3283ef)

Co-authored-by: Inada Naoki <songofacandy@gmail.com>

Doc/library/exceptions.rst

@@ -691,16 +691,13 @@ The following exceptions are used as warning categories; see the
 
 .. exception:: PendingDeprecationWarning
 
-   Base class for warnings about features which will be deprecated in the
-   future.
+   Base class for warnings about features which are obsolete and
+   expected to be deprecated in the future, but are not deprecated
+   at the moment.
 
-   .. 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.
+   This class is rarely used as emitting a warning about a possible
+   upcoming deprecation is unusual, and :exc:`DeprecationWarning`
+   is preferred for already active deprecations.
 
 
 .. exception:: SyntaxWarning