doc: fix capture action name from 'list' -> 'get'.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
dabba: split action from command help handling.

The usual dabba invocation syntax is:
  $ dabba <command> [<subcommand>] <action>

When 'help' is called from a command, it must print the command
manpages. (e.g dabba help capture)

However, when '--help' is called from an action it must print the
action available options. (e.g. dabba capture get --help)

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>