Improve -d and -n documentation

Improve -d and -n documentation

* Adjust `fping.pod` entries for `-d, --rdns` and `-n, --name`:

  * The code performs reverse DNS lookups on the given targets,
    not on source addresses from ICMP Echo Responses or ICMP
    error messages.  Adjust the `-d` description to reflect this.

  * The sentence describing `-n` seems incomplete, so provide a
    completion in similar spirit to `-d`.

* Add `-d, --rdns` to `-h, --help` output since `-d` and `-n` are
  no longer equivalent.