Clarify interaction with watchdog API

[uebayasi]

watchdog has to be stopped just before reboot(9) is called from either normal shutdown code path or emergent code path (panic(9)). In the latter case, watchdog has to be stopped in polling manner.

Watchdog drivers using a serial controller (including ipmi(4)) have to be careful about it; drivers have to provide both sleeping and polling access modes. The former is to avoid unnecessary busy polling in normal operation. The latter is to access controller under situation without interrupt ("cold" mode).

Clarify these and document somewhere.