watchdog: update/improve/consolidate watchdog driver

Move the limited watchdog driver help from kernel-parameters.txt
to Documentation/watchdog/watchdog-parameters.txt and add info to it
for all watchdog drivers except the ones that have driver-specific
files already.

Correct minor comments and MODULE_PARM_DESC() text in 2 places.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>