4.1 migration notes

[shilman]

Issue: N/A

What I did

Document minor migration notes for 4.1.

How to test

N/A

[codecov]

Codecov Report

Merging #5194 into next will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             next    #5194      +/-   ##
==========================================
- Coverage   35.22%   35.07%   -0.16%     
==========================================
  Files         597      594       -3     
  Lines        7418     7362      -56     
  Branches     1016     1007       -9     
==========================================
- Hits         2613     2582      -31     
+ Misses       4294     4271      -23     
+ Partials      511      509       -2

Impacted Files	Coverage Δ
addons/knobs/src/components/PropForm.js	30.76% <0%> (-53.85%)	⬇️
addons/knobs/src/components/Panel.js	63.88% <0%> (-17.7%)	⬇️
addons/info/src/components/types/Shape.js	21.42% <0%> (-7.15%)	⬇️
addons/info/src/components/PropTable.js	96% <0%> (-0.78%)	⬇️
...b/core/src/server/preview/iframe-webpack.config.js	0% <0%> (ø)	⬆️
...ents/stories_panel/stories_tree/tree_decorators.js	100% <0%> (ø)	⬆️
addons/actions/src/lib/decycle.js	81.39% <0%> (ø)	⬆️
addons/actions/src/lib/reviver.js	100% <0%> (ø)	⬆️
lib/core/src/server/manager/manager-config.js	0% <0%> (ø)	⬆️
lib/core/src/server/manager/manager-preset.js	0% <0%> (ø)	⬆️
... and 17 more

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