growfs(7): conditionalize mention of adding dump device

The growfs_fstab script will add a new swap partition as a dump
device as well, but only if dumpdev is set to AUTO.

Reviewed by: pauamma
Differential Revision: https://reviews.freebsd.org/D38166

(cherry picked from commit 7d189f669836b81dbdeb7bc7178040a43ab9d1fe)