Bug #1337990: clarified that `doctest` does not support examples

requiring both expected output and an exception.

I'll backport to 2.4 next.

Doc/lib/libdoctest.tex

@@ -407,10 +407,13 @@ You can force use of your own dict as the execution context by passing
 \subsubsection{What About Exceptions?\label{doctest-exceptions}}
 
 No problem, provided that the traceback is the only output produced by
-the example:  just paste in the traceback.  Since tracebacks contain
-details that are likely to change rapidly (for example, exact file paths
-and line numbers), this is one case where doctest works hard to be
-flexible in what it accepts.
+the example:  just paste in the traceback.\footnote{Examples containing
+    both expected output and an exception are not supported.  Trying
+    to guess where one ends and the other begins is too error-prone,
+    and that also makes for a confusing test.}
+Since tracebacks contain details that are likely to change rapidly (for
+example, exact file paths and line numbers), this is one case where doctest
+works hard to be flexible in what it accepts.
 
 Simple example:
 

Misc/NEWS

@@ -171,6 +171,8 @@ Tools
 Documentation
 -------------
 
+- Bug #1337990: clarified that ``doctest`` does not support examples
+  requiring both expected output and an exception.
 
 
 What's New in Python 2.5 alpha 1?