Change dcsr.cause priorities (#693)

* Clarify dcsr.cause priorities for #691

* Apply suggestions from code review

Co-authored-by: Tim Newsome <tim@sifive.com>

* Review feedback: Incorporate table 5.2 by reference.

* Minor wording change

Co-authored-by: Tim Newsome <tim@sifive.com>

* Recommend the new dcsr.cause priority but allow the old priority.

* Explain why dcsr.cause priorities can vary

Co-authored-by: Tim Newsome <tim@sifive.com>

Co-authored-by: Tim Newsome <tim@sifive.com>