cli/inbox: Clearify "clear" behaviour — heartwood

This patch clearifies (in the help message of the command) how the “clear” subcommand behaves when passing ids.

     rad inbox [<option>...]
     rad inbox list [<option>...]
     rad inbox show <id> [<option>...]
     rad inbox clear [<option>...]
     rad inbox clear <id...> [<option>...]
     By default, this command lists all items in your inbox.
     If your working directory is a Radicle repository, it only shows item

     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.
     The `rad inbox clear` command will delete all notifications by their passed id
     or all notifications if no ids were passed.
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