Ensure that TX IRQs are prioritised over RX IRQs in sDDF, and add documentation about this.

[Courtney3141]

I already have an open PR for fixing this in the network subsystem, PR #408. I'll ensure to add this in the developer docs and design docs.

I'll also make sure this is true for serial as well.

Others should check the other subsystems.

[Ivan-Velickovic]

I think serial is the only other device class to consider, all other systems don't have a notion of RX vs TX.