`SerialUSB` object not documented

[edmundsj]

It appears there is no official documentation for the SerialUSB library for the Arduino Due. Even if it's a carbon copy of the Serial() library (it's not, the baud rate doesn't mean anything, the physical port is different), it would be very helpful to add this to the docs.

[per1234]

This information was documented at one time, but then was removed without explanation in ead1ebe