#2986: document SequenceMatcher heuristic.
diff --git a/Doc/library/difflib.rst b/Doc/library/difflib.rst
index d749e14..9cd76e3 100644
--- a/Doc/library/difflib.rst
+++ b/Doc/library/difflib.rst
@@ -37,6 +37,11 @@
complicated way on how many elements the sequences have in common; best case
time is linear.
+ **Heuristic:** To speed-up matching, items that 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 a
+ future version.
.. class:: Differ