Add documentation for concurrency helpers

[thokari]

This is for #1058 , please let me know if this is what you expect, then I will wrap it up for the other classes. Also the indentation looks a bit strange for the number hints on the AgeFilter example, I'd fix that as well if you don't mind.

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[codecov[bot]]

Codecov Report

Merging #1173 (05e6d3e) into master (b8bfe54) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1173   +/-   ##
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  Coverage     76.52%   76.52%           
- Complexity     3754     3756    +2     
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  Files           405      405           
  Lines         11437    11437           
  Branches       1396     1395    -1     
=========================================
  Hits           8752     8752           
  Misses         2197     2197           
  Partials        488      488           

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[winne42]

@leonard84 , @Vampire , @thokari Will this ever be merged? Testing concurrency is always hard, insufficient documentation makes it even harder.