Clarify docs of DeviceCapabilities MTU

[Dirbaio]

The "MTU" in smoltcp is the max byte count that can be received/sent by the Device, including all headers. This means that it includes the Ethernet header, it's the "Ethernet MTU".

In contrast, Linux's "MTU" is the "IP MTU", not counting the Ethernet header.

This often creates confusion, the docs could be clarified to address this.