Issue #13121: Support in-place math operators for collections.Counter().
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst
index f4edced..c9e3864 100644
--- a/Doc/library/collections.rst
+++ b/Doc/library/collections.rst
@@ -293,7 +293,7 @@
     Counter({'b': 4})
 
 .. versionadded:: 3.3
-   Added support for unary plus and unary minus.
+   Added support for unary plus, unary minus, and in-place multiset operations.
 
 .. note::