ctl.conf(5): fix LUN size in UCL format example.

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