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: NHidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: NAndi Kleen <ak@linux.intel.com>
Signed-off-by: NHidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>

Documentation/x86/x86_64/boot-options.txt

@@ -5,6 +5,8 @@ only the AMD64 specific ones are listed here.
 
 Machine check
 
+   Please see Documentation/x86/x86_64/machinecheck for sysfs runtime tunables.
+
    mce=off disable machine check
    mce=bootlog Enable logging of machine checks left over from booting.
                Disabled by default on AMD because some BIOS leave bogus ones.

Documentation/x86/x86_64/machinecheck

@@ -41,7 +41,9 @@ check_interval
 	the polling interval.  When the poller stops finding MCEs, it
 	triggers an exponential backoff (poll less often) on the polling
 	interval. The check_interval variable is both the initial and
-	maximum polling interval.
+	maximum polling interval. 0 means no polling for corrected machine
+	check errors (but some corrected errors might be still reported
+	in other ways)
 
 tolerant
 	Tolerance level. When a machine check exception occurs for a non