Remove quotes around size in the LUN section and change the suffix to 'GB'. The
UCL format does recognize 'G' on its own, which uses a base 10 multiplier where
'GB' uses a 2 power multiplier.
Document the difference between valid suffixes when using ctl.conf(5) in the
general syntax form or in UCL format.
Reviewed by: kevans, mav
Approved by: kevans (mentor)
Differential Revision: https://reviews.freebsd.org/D26716