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>