Document return type of date knob (see #1489)

[grncdr]

Issue:

What I did

Updated documentation of date knob according to the details in #1489

How to test

Is this testable with jest or storyshots?

No

Does this need a new example in the kitchen sink apps?

No

Does this need an update to the documentation?

No

[codecov]

Codecov Report

Merging #2332 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2332   +/-   ##
=======================================
  Coverage   21.19%   21.19%           
=======================================
  Files         283      283           
  Lines        6156     6156           
  Branches      722      736   +14     
=======================================
  Hits         1305     1305           
+ Misses       4296     4271   -25     
- Partials      555      580   +25

Impacted Files	Coverage Δ
app/vue/src/server/config/babel.js	0% <0%> (-100%)	⬇️
app/vue/src/server/babel_config.js	0% <0%> (-77.42%)	⬇️
app/vue/src/server/utils.js	0% <0%> (-53.58%)	⬇️
.../src/manager/containers/CommentsPanel/dataStore.js	34.04% <0%> (ø)	⬆️
app/react-native/src/bin/storybook-build.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/init_panels.js	25% <0%> (ø)	⬆️
addons/a11y/src/components/Report/Info.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/shortcuts/actions/shortcuts.js	6.25% <0%> (ø)	⬆️
addons/events/src/components/Event.js	0% <0%> (ø)	⬆️
app/react/src/client/preview/element_check.js	41.17% <0%> (ø)	⬆️
... and 24 more

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[Hypnosphi]

Great, thanks!