gh-152798: update sys.thread_info.lock documentation to match implementation (#153263)

Author: Wojusensei

Doc/library/sys.rst

@@ -2240,8 +2240,9 @@ always available. Unless explicitly noted otherwise, all variables are read-only
 
       The name of the lock implementation:
 
-      * ``"semaphore"``: a lock uses a semaphore
-      * ``"mutex+cond"``: a lock uses a mutex and a condition variable
+      * ``"semaphore"``: a lock uses a semaphore (Python 3.14 and older)
+      * ``"mutex+cond"``: a lock uses a mutex and a condition variable (Python 3.14 and older)
+      * ``"pymutex"``: a lock uses the :c:type:`PyMutex` implementation (Python 3.15 and newer)
       * ``None`` if this information is unknown
 
    .. attribute:: thread_info.version