feat(turncat): Extend help message in `--help`

[levaitamas]

turncat -h output had and extra pflag: help requested line. This PR removes it, and provides the extended help message text.

Output of ./turncat -h:

Before

Usage of turncat:
  -i, --insecure     Insecure TLS mode, accept self-signed certificates (default: false).
  -l, --log string   Log level (default: all:WARN). (default "all:WARN")
  -v, --verbose      Verbose logging, identical to -l all:DEBUG.
pflag: help requested

After

turncat [-l|--log <level>] [-i|--insecure] client server peer
        client: <udp|tcp|unix>://<listener_addr>:<listener_port>
        server: <turn://<auth>@<server_addr>:<server_port> | <k8s://<namesspace>/<name>:listener
        peer: udp://<peer_addr>:<peer_port>
        auth: <username:password|secret>
  -h, --help         Display this help text and exit
  -i, --insecure     Insecure TLS mode, accept self-signed certificates (default: false).
  -l, --log string   Log level (default: all:WARN). (default "all:WARN")
  -v, --verbose      Verbose logging, identical to -l all:DEBUG.