bectl(8): add description for create subcommand

In commit r345845, a portion of documentation for the create subcommand was
removed. Specifically, for creating a snapshot of an existing boot
environment. bectl even has a test-case for this functionality.

Removing the sub-command description was discussed in PR 235850.

This patch brings back the second "create" description that was originally
in place. Albeit, with a few wording/clarifying changes.

Submitted by: Rob Fairbanks <rob.fx907 gmail com>
Reviewed by: kevans
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D20249