Add a DeprecationWarning for when warnings.showwarning() is set to a function
that lacks support for the new 'line' argument.
diff --git a/Doc/library/warnings.rst b/Doc/library/warnings.rst
index 94d4738..87c4fd6 100644
--- a/Doc/library/warnings.rst
+++ b/Doc/library/warnings.rst
@@ -220,7 +220,8 @@
``warnings.showwarning``.
.. versionchanged:: 2.6
- Added the *line* argument.
+ Added the *line* argument. Implementations that lack the new argument
+ will trigger a :exc:`DeprecationWarning`.
.. function:: formatwarning(message, category, filename, lineno[, line])