Merge in all documentation changes since branching 3.4.0rc1.
diff --git a/Doc/library/smtplib.rst b/Doc/library/smtplib.rst
index eba8ae9..ec8dc9d 100644
--- a/Doc/library/smtplib.rst
+++ b/Doc/library/smtplib.rst
@@ -117,6 +117,9 @@
    Subclass of :exc:`OSError` that is the base exception class for all
    the other exceptions provided by this module.
 
+   .. versionchanged:: 3.4
+      SMTPException became subclass of :exc:`OSError`
+
 
 .. exception:: SMTPServerDisconnected