Improve documentation about difference between riss and niss.

[ablack3]

The difference between riss and niss is not well documented.

"The ISS was described as the sum of the squares of the highest Abbreviated Injury Scale (AIS) grade in each of the three most severely injured body regions (head/neck, face, thorax, abdomen, extremities, and external) with possible values ranging from 1 to 75. The patient with an AIS of 6 was automatically assigned an ISS value of 75, which was associated with almost certain mortality."

"NISS for each patient was computed based on the three most severe injuries sustained regardless of body region as described by AIS-90 terminology."

Here is a reference: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3400229/