[DOC] Add MWE doctest to ml module

[gahjelle]

PR Description

Please describe the changes proposed in the pull request:

• Add MWE doctest to ml module

This PR relates to #972.

PR Checklist

Please ensure that you have done the following:

1. PR in from a fork off your branch. Do not PR from <your_username>:dev, but rather from <your_username>:<feature-branch_name>.

2. If you're not on the contributors list, add yourself to AUTHORS.md.

3. Add a line to CHANGELOG.md under the latest version header (i.e. the one that is "on deck") describing the contribution.
   • Do use some discretion here; if there are multiple PRs that are related, keep them in a single line.

Automatic checks

There will be automatic checks run on the PR. These include:

• Building a preview of the docs on Netlify
• Automatically linting the code
• Making sure the code is documented
• Making sure that all tests are passed
• Making sure that code coverage doesn't go down.

Relevant Reviewers

Please tag maintainers to review.

• @ericmjl

[codecov]

Codecov Report

Merging #1085 (d2a57f3) into dev (b9be94d) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev    #1085      +/-   ##
==========================================
- Coverage   97.75%   97.72%   -0.04%     
==========================================
  Files          77       77              
  Lines        3121     3121              
==========================================
- Hits         3051     3050       -1     
- Misses         70       71       +1