bectl.8: Describe better

+ Concise document description for consistency and apropos results
+ Improve introductory paragraph, mentioning boot loader support
+ Explain -r in "Supported Subcommands and Flags"
+ Clarify the purpose of the check subcommand
+ Add two basic examples, creating and mounting
+ Fold some long lines, correct a stray capitalization.

MFC after: 3 days
Co-authored-by: kevans

(cherry picked from commit d7baf5f70802a9045f5792a7063b2614bf17356a)