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

(cherry picked from commit fdafa1d0ed0a8930b52ee81e57c931cc4d5c2388)

Co-authored-by: idomic <michael.ido@gmail.com>
diff --git a/Doc/library/threading.rst b/Doc/library/threading.rst
index fd815ef..93ea4bd 100644
--- a/Doc/library/threading.rst
+++ b/Doc/library/threading.rst
@@ -488,6 +488,10 @@
 
       There is no return value.
 
+   .. method:: locked()
+      Return true if the lock is acquired.
+
+
 
 .. _rlock-objects: