ARC: Comment on unimplemented CSR behavior.

After a very long e-mail discussion, we landed on maintaining
unimplemented CSR behavior as-is. Just add a quick note to be explicit
about that intention. The end of the discussion is basically:
1. If Sdtrig CSRs are accessed when they are not implemented or Sdtrig
   is not implemented, the hart will trap. This is to maintain backwards
   compatibility with existing implementations that use this for
   discovery.
2. If Sdext CSRs are accessed when they are not implemented, the hart
   will trap. (No need to specify behavior when Sdext is not
   implemented, because in that case Debug Mode doesn't exist and the
   CSRs aren't accessible anyway.) This is to be consistent with Sdtrig
   behavior.

Sdtrig.tex

@@ -358,4 +358,12 @@ \section{Trigger Registers}
 Attempts to access an unimplemented Trigger Register raise an illegal instruction
 exception.
 
+\begin{commentary}
+    Privileged spec v1.12 requires accesses to unimplemented CSRs to raise
+    illegal instruction exceptions, but that may change in the future. Even if
+    it does change, the behavior here overrides it for Sdtrig CSRs for backwards
+    compatibility. This is even true if Sdtrig is not implemented at all, so the
+    discovery algorithm in Section~\ref{sec:trigger} still works.
+\end{commentary}
+
 \input{hwbp_registers.tex}