Update documentation for parameter *notrigger* in einj.txt

Add description of parameter notrigger in the einj.txt.
One can utilize this new parameter to do some SRAR injection
test. Pay attention, the operation is highly depended on the
BIOS implementation. If no proper BIOS supports it, even if
enabling this parameter, expected result will not happen.

v2:
  Update the documentation suggested by Tony

Suggested-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Chen Gong <gong.chen@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>