MB-9472: adjusted ulimit warning text

To note that couchbase user is only for normal (root) installs.

Change-Id: I29d760872081b6824cb75fa93951840f876ee587
Reviewed-on: http://review.couchbase.org/30223
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>