minor #21173 [Messenger] Fix confusing wording about redeliver_timeout duration (OskarStark)

This PR was merged into the 6.4 branch.

Discussion
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[Messenger] Fix confusing wording about `redeliver_timeout` duration

Fix #20963

Changed "slowest message duration" to "longest message duration" to clarify that redeliver_timeout should be greater than the maximum processing time of any message.

Commits
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dd45539 Fix confusing wording about redeliver_timeout duration

Author: javiereguiluz

messenger.rst

@@ -1601,7 +1601,7 @@ The transport has a number of options:
 
     .. note::
 
-        Set ``redeliver_timeout`` to a greater value than your slowest message
+        Set ``redeliver_timeout`` to a greater value than your longest message
         duration. Otherwise, some messages will start a second time while the
         first one is still being handled.