Issue 2986: document heuristic for 3.1
diff --git a/Doc/library/difflib.rst b/Doc/library/difflib.rst
index 433764c..c9d1eee 100644
--- a/Doc/library/difflib.rst
+++ b/Doc/library/difflib.rst
@@ -35,6 +35,11 @@
complicated way on how many elements the sequences have in common; best case
time is linear.
+ **Heuristic:** To speed-up matching, items whose duplicates appear more than 1% of
+ the time in sequences of at least 200 items are treated as junk. This has the
+ unfortunate side-effect of giving bad results for sequences constructed from
+ a small set of items. An option to turn off the heuristic will be added to
+ Python 3.2.
.. class:: Differ