Document trap precision

[imphil]

The RISC-V spec gives us some flexibility how precise traps should be. We need to document our current behavior.

I'd expect all traps to be precise, but we should double-check with the code.

[GregAC]

There have been some fairly recent updates to the RISC-V privileged spec which provide some clarity on behavior here. We should check Ibex complies with the latest version.