Add docs

Author: encukou

Doc/library/exceptions.rst

@@ -429,7 +429,9 @@ The following exceptions are the exceptions that are usually raised.
 
    * Creating a new Python thread.
    * :meth:`Joining <threading.Thread.join>` a running daemon thread.
-   * :func:`os.fork`.
+   * :func:`os.fork`,
+   * acquiring a lock such as :cls:`threading.Lock`, when it is known that
+     the operation would otherwise deadlock.
 
    See also the :func:`sys.is_finalizing` function.
 
@@ -440,6 +442,11 @@ The following exceptions are the exceptions that are usually raised.
 
       :meth:`threading.Thread.join` can now raise this exception.
 
+   .. versionchanged:: next
+
+      This exception may be raised when acquiring :meth:`threading.Lock`
+      or :meth:`threading.RLock`.
+
 .. exception:: RecursionError
 
    This exception is derived from :exc:`RuntimeError`.  It is raised when the

Misc/NEWS.d/next/Library/2025-06-27-09-26-04.gh-issue-87135.33z0UW.rst

@@ -0,0 +1,3 @@
+Acquiring a :cls:`threading.Lock` or :cls:`threading.RLock` at interpreter
+shutdown will raise :exc:`PythonFinalizationError` if Python can determine
+that it would otherwise deadlock.