Add notes and changelog for breaking change to gherkin writer helpers

[luke-hill]

Description

Please include a summary of the change, mentioning any issues that are fixed (or partially fixed) by this change.

Please also include relevant motivation and context.

e.g. "Fixes issue with XYZ"

Type of change

Please delete options that are not relevant.

• Refactoring (improvements to code design or tooling that don't change behaviour)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new behaviour)
• Breaking change (will cause existing functionality to not
  work as expected)

Please add an entry to the relevant section of CHANGELOG.md as part of this pull request.

Note to other contributors

If your change may impact future contributors, explain it here, and remember to update README.md and CONTRIBUTING.md accordingly.

Checklist:

Your PR is ready for review once the following checklist is
complete. You can also add some checks if you want to.

• Tests have been added for any changes to behaviour of the code
• New and existing tests are passing locally and on CI
• bundle exec rubocop reports no offenses
• CHANGELOG.md has been updated