bpo-35821: Add an example to Logger.propagate documentation. (GH-29841)

Doc/library/logging.rst

@@ -80,6 +80,15 @@ is the module's name in the Python package namespace.
       If this evaluates to false, logging messages are not passed to the handlers
       of ancestor loggers.
 
+      Spelling it out with an example: If the propagate attribute of the logger named
+      `A.B.C` evaluates to true, any event logged to `A.B.C` via a method call such as
+      `logging.getLogger('A.B.C').error(...)` will [subject to passing that logger's
+      level and filter settings] be passed in turn to any handlers attached to loggers
+      named `A.B`, `A` and the root logger, after first being passed to any handlers
+      attached to `A.B.C`. If any logger in the chain `A.B.C`, `A.B`, `A` has its
+      `propagate` attribute set to false, then that is the last logger whose handlers
+      are offered the event to handle, and propagation stops at that point.
+
       The constructor sets this attribute to ``True``.
 
       .. note:: If you attach a handler to a logger *and* one or more of its