gh-126548: Add a thread-unsafety warning for importlib.reload() (GH-136704)

Author: bswck

Doc/library/importlib.rst

@@ -206,6 +206,10 @@ Functions
        :exc:`ModuleNotFoundError` is raised when the module being reloaded lacks
        a :class:`~importlib.machinery.ModuleSpec`.
 
+   .. warning::
+      This function is not thread-safe. Calling it from multiple threads can result
+      in unexpected behavior. It's recommended to use the :class:`threading.Lock`
+      or other synchronization primitives for thread-safe module reloading.
 
 :mod:`importlib.abc` -- Abstract base classes related to import
 ---------------------------------------------------------------