document threading.Lock.locked() (GH-17427)

diff --git a/Doc/library/threading.rst b/Doc/library/threading.rst
index 9a68491..96989bd 100644
--- a/Doc/library/threading.rst
+++ b/Doc/library/threading.rst
@@ -496,6 +496,10 @@
 
       There is no return value.
 
+   .. method:: locked()
+      Return true if the lock is acquired.
+
+
 
 .. _rlock-objects: