`SerialUSB` object not documented

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.

Start by reviewing the existing Serial() reference and the SerialUSB library for the Arduino Due. Document the baud-rate behavior and physical-port differences identified in the issue, using the project's reference conventions. Done means SerialUSB has official documentation covering these distinctions.

Written by the indexing model from the issue text.