Documentation of the code

I am trying to modify some buffer properties on the arduino_due_x core (basically, I would like to be able to set the size of each RX TX buffer separately). I find it difficult to understand and modify the code as it is little documented. If somebody has a good knowledge of the core code, could you spend a little time to comment the code?

Start by locating the arduino_due_x core and the code defining its RX and TX buffer properties. Read how those buffers are configured, then confirm with maintainers which areas need comments; done means the relevant core code is documented well enough to support separate RX and TX buffer sizing.

Written by the indexing model from the issue text.