Document makeDecorators for addons authors

[tmeasday]

Issue:

We added a makeDecorators API and use it everywhere but it is not documented.

What I did

Added a allowDeprecatedUsage (defaults to false) option to makeDecorator and set it to true for our addons.

Updated the writing addons documentation to reflect the above.

How to test

Added tests for the API change. Please read the docs.

[tmeasday]

Hey! Looks like the tests actually saved me here

[codecov]

Codecov Report

Merging #3831 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3831      +/-   ##
==========================================
+ Coverage   41.35%   41.38%   +0.02%     
==========================================
  Files         455      455              
  Lines        5184     5186       +2     
  Branches      900      901       +1     
==========================================
+ Hits         2144     2146       +2     
  Misses       2496     2496              
  Partials      544      544

Impacted Files	Coverage Δ
addons/backgrounds/src/index.js	0% <ø> (ø)	⬆️
addons/viewport/src/preview/withViewport.js	28.57% <ø> (ø)	⬆️
addons/info/src/index.js	79.16% <ø> (ø)	⬆️
addons/notes/src/index.js	83.33% <ø> (ø)	⬆️
addons/knobs/src/index.js	0% <ø> (ø)	⬆️
lib/addons/src/make-decorator.js	94.44% <100%> (+0.69%)	⬆️

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