[IA64] Document the "nomca" boot parameter

"nomca" can be used to disable machine check handling
Signed-Off-By: Horms <horms@verge.net.au>
Signed-off-by: Tony Luck <tony.luck@intel.com>

Documentation/kernel-parameters.txt

@@ -1034,6 +1034,8 @@ running once the system is up.
 
 	nomce		[IA-32] Machine Check Exception
 
+	nomca		[IA-64] Disable machine check abort handling
+
 	noresidual	[PPC] Don't use residual data on PReP machines.
 
 	noresume	[SWSUSP] Disables resume and restores original swap