Update auth_token_validity_seconds description
Update auth_token_validity_seconds description

The caveats are no longer relevant since KUDU-2013 landed.

Change-Id: I050e39d0377049cdaac0afb5085a8a9a19d620a5
Reviewed-on: http://gerrit.cloudera.org:8080/8413
Tested-by: Kudu Jenkins
Reviewed-by: Alexey Serbin <864f6b82bd94e8dda64141d4274519ee66e0102e@cloudera.com>

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