[docs] change the parallelism tutorial to spawn only a limited set of tasks

[odow]

x-ref #4150 (comment)

We probably shouldn't be encouraging people to spawn a very large set of tasks. We might also consider pointing users to https://juliafolds2.github.io/OhMyThreads.jl/stable/ instead of manually rolling their own like this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.91%. Comparing base (c2d7fe7) to head (0e6c144).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4164   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files          42       42           
  Lines        6229     6229           
=======================================
  Hits         6224     6224           
  Misses          5        5           

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