[docs] add an example to the Parallelism tutorial about channels

[odow]

Closes #4150

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.91%. Comparing base (7f3a9f5) to head (058886d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4155   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files          42       42           
  Lines        6229     6229           
=======================================
  Hits         6224     6224           
  Misses          5        5           

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