API CI Lint docs unclear on support for "extends" for merged YAML

Problem to solve

#258843 (closed) says that extends is not yet supported for merged YAML configuration, but it seems it already is.

The documentation at https://docs.gitlab.com/ee/api/lint.html#yaml-expansion says

The CI lint returns an expanded version of the configuration. The expansion does not work for CI configuration added with include: local, or with extends:.

This example from the blog post at https://about.gitlab.com/blog/2021/04/21/devops-workflows-json-format-jq-ci-cd-lint/ works fine:

.job-tmpl:
  image: alpine:latest
  variables:
    BUILD_TYPE: "Debug"
  script:
    - echo "Hello from GitLab 🦊"

test-extends-stage:
  extends: .job-tmpl

test-extends-prod:
  extends: .job-tmpl
  variables:
    BUILD_TYPE: "Release"
  script:
    - echo "Hello from GitLab 🦊🌈"
$ jq --null-input --arg yaml "$(<.gitlab-ci.main.yml)" '.content=$yaml' \
| curl "https://gitlab.com/api/v4/ci/lint?include_merged_yaml=true" \
--header 'Content-Type: application/json' --data @- \
| jq --raw-output '.merged_yaml'
---
".job-tmpl":
  image: alpine:latest
  variables:
    BUILD_TYPE: Debug
  script:
  - "echo \"Hello from GitLab \U0001F98A\""
test-extends-stage:
  image: alpine:latest
  variables:
    BUILD_TYPE: Debug
  script:
  - "echo \"Hello from GitLab \U0001F98A\""
  extends: ".job-tmpl"
test-extends-prod:
  image: alpine:latest
  variables:
    BUILD_TYPE: Release
  script:
  - "echo \"Hello from GitLab \U0001F98A\U0001F308\""
  extends: ".job-tmpl"

Further details

Potentially this was solved with #231352 (closed) though needs engineering / product validation from @matteeyah @dhershkovitch @jreporter

This was originally reported at the jq / CI/CD blog post I wrote a while ago: https://about.gitlab.com/blog/2021/04/21/devops-workflows-json-format-jq-ci-cd-lint/?utm_medium=social&utm_source=linkedin&utm_campaign=blog#comment-5372074958

Proposal

Remove the extends limitation in the API docs or specify what exactly does not work with YAML expansion.

Who can address the issue

Technical writers and engineers.

Other links/references

cc @marcel.amirault