Add host MTU settings reqs to docs

Description

By request of Avinash Sridharan from customer slack channel:

```
the default configuration of the overlay chooses an MTU of 1420
https://github.com/dcos/dcos/blob/master/gen/calc.py#L392
Assuming an interface MTU on the host of 1500 bytes
So ideally you shouldn't need to change anything. My apologies for missing this in the documentation, and thanks for bringing this up.
```

Assignee

Avinash Sridharan

Components

Configure