Relevant page: http://test.docs.10gen.cc/master/release-notes/2.6-upgrade/
The sentence "If successful, mongos --upgrade will exit with the following messages:" is incorrect. mongos will not output anything. There are similar messages in the log to the one referenced here, but they are not actually output. This is particularly confusing in that later in the same section the documentation states: "If mongos fails to start, check the log for more information."
Additionally, we may want to make it more clear to the reader that a mongos started with --upgrade will not actually start a mongos instance. It will perform the upgrade operation and then exit. I found this confusing/surprising so I think making this more clear in the documentation will be useful.