cli: document `rad inbox show` — heartwood

The documentation for rad inbox show was missing.

Add documentation into the help message.

     rad inbox [<option>...]
     rad inbox list [<option>...]
     rad inbox show <id> [<option>...]
     rad inbox clear [<option>...]
     By default, this command lists all items in your inbox.
     If your working directory is a Radicle repository, it only shows item
     belonging to this repository, unless `--all` is used.
     The `rad inbox show` command takes a notification ID (which can be found in
     the `list` command) and displays the information related to that
     notification. This will mark the notification as read.
     The `rad inbox clear` command will delete all notifications in the inbox.
 Options
     --all                Operate on all repositories