Update custom webpack config docs

[leonaves]

As of f422481, the app will not load without the first two plugins in the config, so this should no longer read: "you should try to"; the preservation of those plugins is now a necessity, so I have added it to the list of "make sure to preserve"s.

[codecov]

Codecov Report

Merging #2578 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2578   +/-   ##
=======================================
  Coverage   32.68%   32.68%           
=======================================
  Files         398      398           
  Lines        8838     8838           
  Branches      959      947   -12     
=======================================
  Hits         2889     2889           
- Misses       5283     5293   +10     
+ Partials      666      656   -10

Impacted Files	Coverage Δ
...ents/stories_panel/stories_tree/tree_decorators.js	34.37% <0%> (ø)	⬆️
addons/knobs/src/components/types/Array.js	34.14% <0%> (ø)	⬆️
addons/actions/src/lib/reviver.js	58.33% <0%> (ø)	⬆️
addons/graphql/src/components/FullScreen/index.js	0% <0%> (ø)	⬆️
...res__/update-addon-info/update-addon-info.input.js	0% <0%> (ø)	⬆️
addons/jest/src/components/Indicator.js	0% <0%> (ø)	⬆️
app/react-native/src/bin/storybook.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/types/Number.js	7.93% <0%> (ø)	⬆️
lib/components/src/table/cell.js	65.21% <0%> (ø)	⬆️
addons/actions/src/lib/types/function/index.js	77.77% <0%> (ø)	⬆️
... and 47 more

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