whatsnew: expand 'dis' entry.

Also add one missing versionadded.
diff --git a/Doc/library/dis.rst b/Doc/library/dis.rst
index 8293d49..d86550f 100644
--- a/Doc/library/dis.rst
+++ b/Doc/library/dis.rst
@@ -40,6 +40,8 @@
 Bytecode analysis
 -----------------
 
+.. versionadded:: 3.4
+
 The bytecode analysis API allows pieces of Python code to be wrapped in a
 :class:`Bytecode` object that provides easy access to details of the
 compiled code.