bectl.8: Clean up & clarify the create subcommand

• Improve formatting
• Use consistent variable names
• Improve the description of the create subcommand (1)

PR: 235850 (1)
Submitted by: kevans (1)
Reported by: ler (1)
Reviewed by: kevans
Approved by: src (kevans)
Differential Revision: https://reviews.freebsd.org/D19666