Clarify action=1 (enter Debug Mode) with dmode=0 (#501)

* Recommend clearing action when dmode is cleared.

Otherwise you might end up in a situation where regular software can
control when to enter debug mode.

* Recommend hardware ignore illegal triggers.

* Change to use WARL.

Hopefully I found some language that makes sense when csrci is used to
clear just dmode, as well as writing a brand new value, and only changes
action if it's actually necessary.