Include additional "--help" info for setting water marks

Include information for cbepctl, that tells the user
that if the argument for the water marks is suffixed with
a '%' then it would assume the percentage of the RAM quota.

Change-Id: I3d302b9142ab682645e71dd6dd7217d654b8935f
Reviewed-on: http://review.couchbase.org/43562
Reviewed-by: Sriram Ganesan <sriram@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>