docs(knobs) Add caveat about date defaultValue

[SpainTrain]

Related to #804

Got bit by this and want to help others avoid :-)

[codecov]

Codecov Report

Merging #1719 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1719   +/-   ##
=======================================
  Coverage   21.18%   21.18%           
=======================================
  Files         252      252           
  Lines        5694     5694           
  Branches      690      677   -13     
=======================================
  Hits         1206     1206           
- Misses       3956     3979   +23     
+ Partials      532      509   -23

Impacted Files	Coverage Δ
...res__/update-addon-info/update-addon-info.input.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/layout.js	12.5% <0%> (ø)	⬆️
...rc/modules/ui/components/left_panel/text_filter.js	30.98% <0%> (ø)	⬆️
addons/info/src/components/markdown/htags.js	30% <0%> (ø)	⬆️
addons/knobs/src/components/PropForm.js	8.51% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/menu_item.js	19.14% <0%> (ø)	⬆️
...codemod/src/transforms/update-organisation-name.js	40.62% <0%> (ø)	⬆️
addons/knobs/src/components/types/Boolean.js	11.62% <0%> (ø)	⬆️
lib/ui/src/modules/api/configs/init_api.js	40.47% <0%> (ø)	⬆️
lib/components/src/navigation/menu_link.js	0% <0%> (ø)	⬆️
... and 13 more

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[ndelangen]

Thank you @SpainTrain I'm sure this will help people!

[Hypnosphi]

do not do ... date('Label')

This bothers me a bit. If the default behaviour is buggy, we should change it not forbid