bpo-28795: Signal documentation: Fix misleading statement. (GH-13121)

Doc/library/signal.rst

@@ -16,7 +16,8 @@ The :func:`signal.signal` function allows defining custom handlers to be
 executed when a signal is received.  A small number of default handlers are
 installed: :const:`SIGPIPE` is ignored (so write errors on pipes and sockets
 can be reported as ordinary Python exceptions) and :const:`SIGINT` is
-translated into a :exc:`KeyboardInterrupt` exception.
+translated into a :exc:`KeyboardInterrupt` exception if the parent process
+has not changed it.
 
 A handler for a particular signal, once set, remains installed until it is
 explicitly reset (Python emulates the BSD style interface regardless of the