Clarify stoptime behavior. (#769)

* Clarify stoptime behavior.

More accurate state the behavior specified in Section 4.1, so people
don't have to read that in order to really understand this part.

See #766.

* Replace "snap forward" with clearer language

Co-authored-by: Paul Donahue <48959409+pdonahue-ventana@users.noreply.github.com>
Signed-off-by: Tim Newsome <tim@sifive.com>

* Multi-hart implementations will likely wire stoptime=0

Signed-off-by: Tim Newsome <tim@sifive.com>
Co-authored-by: Paul Donahue <48959409+pdonahue-ventana@users.noreply.github.com>