Improve JS docs example

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:
Resolves #3955

Description

Include handling of multiple outputs and errors in example as suggested

[github-actions]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#gh-3955-js-docs

or load it into the REPL:
https://rollupjs.org/repl/?pr=4331

[codecov]

Codecov Report

Merging #4331 (9459da3) into master (2a899d5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4331   +/-   ##
=======================================
  Coverage   98.44%   98.44%           
=======================================
  Files         205      205           
  Lines        7314     7314           
  Branches     2084     2084           
=======================================
  Hits         7200     7200           
  Misses         55       55           
  Partials       59       59           

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