x86, mce: improve documentation

Document that check_interval set to 0 means no polling.
Noticed by Hidetoshi Seto

Also add a reference from boot options to the sysfs tunables

Acked-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>