Add back long help (in a more consistent way)

Today, we have re-routed all four forms of help to only show the short help, the default clap provides on -h.

Instead, let's provide both long and short help, like this:

• Global -h on any command, like rv ruby pin -h: show short help
• Global --help on any command, like rv ruby pin --help: show short help
• rv help [NAME], like rv help ruby pin: show long help
• rv NAME help, like rv ruby help pin or rv ruby pin help: removed, it's confusing to have many help commands