ACPI/APEI: Update einj documentation for param1/param2

To ensure EINJ working well when injecting errors via EINJ
table, add some restrictions: param1 must be a valid physical
RAM address and param2 must specify page granularity or
narrower.
Signed-off-by: Chen Gong <gong.chen@linux.intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>

Documentation/acpi/apei/einj.txt

@@ -47,11 +47,16 @@ directory apei/einj. The following files are provided.
 
 - param1
   This file is used to set the first error parameter value. Effect of
-  parameter depends on error_type specified.
+  parameter depends on error_type specified. For example, if error
+  type is memory related type, the param1 should be a valid physical
+  memory address.
 
 - param2
   This file is used to set the second error parameter value. Effect of
-  parameter depends on error_type specified.
+  parameter depends on error_type specified. For example, if error
+  type is memory related type, the param2 should be a physical memory
+  address mask. Linux requires page or narrower granularity, say,
+  0xfffffffffffff000.
 
 - notrigger
   The EINJ mechanism is a two step process. First inject the error, then