[docs] improve documentation for risk measures

[odow]

Closes #812

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.28%. Comparing base (601472f) to head (e21760c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #813      +/-   ##
==========================================
+ Coverage   96.25%   96.28%   +0.03%     
==========================================
  Files          26       26              
  Lines        3523     3525       +2     
==========================================
+ Hits         3391     3394       +3     
+ Misses        132      131       -1     

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