This page:

http://docs.mongodb.org/manual/tutorial/clear-jumbo-flag/

erroneously claims

If the chunk size no longer hits the limits, MongoDB clears the jumbo flag for the chunk when the mongos reloads or rewrites the chunk metadata.

This is not true.

It looks like DOCS-4163 missed that SERVER-13024 (which it references) is not actually resolved?

This ticket is to request that the above sentence be replaced by one that explains when jumbo is cleared (currently only on successful chunk split), and that this means sometimes it needs to be cleared manually.