Added warning about logging use from asynchronous signal handlers.

Doc/library/logging.rst

@@ -2351,6 +2351,9 @@ needing to be done by its clients. It achieves this though using threading
 locks; there is one lock to serialize access to the module's shared data, and
 each handler also creates a lock to serialize access to its underlying I/O.
 
+Note that in Linux/Unix environments, threading APIs  may not be usable in
+asynchronous signal handlers. This may mean that you cannot use logging from
+within asynchronous signal handlers; please check your OS documentation.
 
 Configuration
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