Remove documentation of what "self" param is

I think it's so common that it's fair to not devote documentation to it.

Signed-off-by: Andy Grover <agrover@redhat.com>
Fix up replicate_volume_range

Clarified some logic that might be confusing. Raise an exception if
argument counts don't match. Flag arguments as 'append'.

Signed-off-by: Andy Grover <agrover@redhat.com>
Customize --help output

Unless we give add_subparser a metavar, it uses a concatenation of all
subcommands, which is not attractive. Other parameters don't seem to be
needed so remove them, and put the description for the main parser
in the instantiation instead of afterwards.

Signed-off-by: Andy Grover <agrover@redhat.com>
Allow both upper and lowercase for types

Because we're on Unix and should allow lowercase if possible.

Improve the command list walking to pass kwargs more easily.

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