Update DRBG docs

[mborland]

Closes: #232

[cppalliance-bot]

An automated preview of the documentation is available at https://241.crypt.prtest2.cppalliance.org/libs/crypt/doc/html/index.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.94%. Comparing base (0139078) to head (b061a33).
Report is 7 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #241   +/-   ##
========================================
  Coverage    99.94%   99.94%           
========================================
  Files           96       96           
  Lines         3711     3711           
========================================
  Hits          3709     3709           
  Misses           2        2           

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