Change some subcommand help descriptions
Change some subcommand help descriptions

Only capitalize first word.

Use every opportunity to teach the user how blockdevs and filesystems
relate to a pool.

Signed-off-by: Andy Grover <b7d524d2f5cc5aebadb6b92b08d3ab26911cde33@redhat.com>

stratis-storage/stratis-cli,stratis-storage/stratis-cli