Add return type documentation for util.have_jax() and is_jax_compatible()

[raghuramshankar]

Description

Fixes #4004

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[brosaplanella]

Thanks, can you please add the description I suggested before we merge?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.58%. Comparing base (62fdd5f) to head (b2aa523).
Report is 1 commits behind head on develop.

❗ Current head b2aa523 differs from pull request most recent head 65ccc23. Consider uploading reports for the commit 65ccc23 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4005   +/-   ##
========================================
  Coverage    99.58%   99.58%           
========================================
  Files          257      257           
  Lines        21242    21242           
========================================
  Hits         21154    21154           
  Misses          88       88           

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