This patch contains the following cleanups:
1. Simplify `ffcli.Command` definitions;
2. Word-wrap help text, consistent with other commands;
3. `tailscale dns --help` usage makes subcommand usage more obvious;
4. `tailscale dns query --help` describes DNS record types.
Updates #cleanup
Signed-off-by: Simon Law <sfllaw@tailscale.com>