[docs] add a section clarifying the usage of the init kwarg

[odow]

Closes #4144

There's a lot more subtlety here than you might expect.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.91%. Comparing base (93952fb) to head (eec09dd).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4146   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files          42       42           
  Lines        6224     6224           
=======================================
  Hits         6219     6219           
  Misses          5        5           

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