Add usage note.
diff --git a/Doc/library/functools.rst b/Doc/library/functools.rst
index 1b0d82a..f5c6608 100644
--- a/Doc/library/functools.rst
+++ b/Doc/library/functools.rst
@@ -49,8 +49,9 @@
Since a dictionary is used to cache results, the positional and keyword
arguments to the function must be hashable.
- If *maxsize* is set to None, the LRU feature is disabled and the cache
- can grow without bound.
+ If *maxsize* is set to None, the LRU feature is disabled and the cache can
+ grow without bound. The LRU feature performs best when *maxsize* is a
+ power-of-two.
If *typed* is set to True, function arguments of different types will be
cached separately. For example, ``f(3)`` and ``f(3.0)`` will be treated