IPMI: documentation fixes

Clean up IPMI documentation to remove references to high-res timers and add
info about the polling thread.  Also fix an doc error for a parameter.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>