fix info not importing docgen descriptions

[danielduan]

Issue:

Addon-Info no longer displays component description comment from react-docgen. Think it might have been broken since we implemented hierarchy.

What I did

fixed it

How to test

Is this testable with jest or storyshots?
yes

Does this need a new example in the kitchen sink apps?
no, existing example should already work

Does this need an update to the documentation?
yes

If your answer is yes to any of these, please make sure to include it in your PR.

[codecov]

Codecov Report

Merging #2133 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #2133   +/-   ##
=======================================
  Coverage   21.42%   21.42%           
=======================================
  Files         263      263           
  Lines        5805     5805           
  Branches      700      686   -14     
=======================================
  Hits         1244     1244           
- Misses       4003     4048   +45     
+ Partials      558      513   -45

Impacted Files	Coverage Δ
addons/info/src/components/Story.js	76.19% <0%> (ø)	⬆️
app/vue/src/server/utils.js	0% <0%> (-53.58%)	⬇️
addons/info/src/components/markdown/htags.js	30% <0%> (ø)	⬆️
addons/info/src/components/PropVal.js	41.41% <0%> (ø)	⬆️
app/react/src/client/preview/error_display.js	0% <0%> (ø)	⬆️
...res__/update-addon-info/update-addon-info.input.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/routes.js	0% <0%> (ø)	⬆️
...rc/modules/ui/components/left_panel/text_filter.js	30.98% <0%> (ø)	⬆️
addons/knobs/src/components/PropForm.js	9.09% <0%> (ø)	⬆️
lib/ui/src/modules/api/actions/api.js	50.47% <0%> (ø)	⬆️
... and 15 more

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