MINOR: config: allow no set-dumpable config option

MINOR: config: allow no set-dumpable config option

in global config parsing, we currently expect to have a possible no
keyword (KWN_NO), but we never allow it in config parsing.
another patch could have been to simply remove the code handling a
possible KWN_NO.
take this opportunity to update documentation of set-dumpable.

Signed-off-by: William Dauchy <w.dauchy@criteo.com>