The Mesos configuration guide documents the `--ip6` and `--ip6_discovery_command` flags incorrectly

The Apache Mesos configuration guide lists the `ip6_discovery_command` and the `ip6` flags being common to both the master and the agent:
http://mesos.apache.org/documentation/latest/configuration/master-and-agent/

However these flags have been introduced only in the agent and not the master:
```
[vagrant@centos8 bin]$ ./mesos-master.sh --help | grep discovery
with `--ip_discovery_command`.
--ip_discovery_command=VALUE Optional IP discovery binary: if set, it is expected to emit
[vagrant@centos8 bin]$ ./mesos-agent.sh --help | grep discovery
--appc_simple_discovery_uri_prefix=VALUE URI prefix to be used for simple discovery of appc images,
with `--ip_discovery_command`.
with '--ip6_discovery_command'.
--ip6_discovery_command=VALUE Optional IPv6 discovery binary: if set, it is expected to emit
--ip_discovery_command=VALUE Optional IP discovery binary: if set, it is expected to emit
[vagrant@centos8 bin]$ cd ~/dev/
[vagrant@centos8 dev]$ ls
```

So we need to fix the documentation.