Document when current coverage configuration option was introduced

* Introduced in v8.17

[skip ci]

changelogs/unreleased/28524-gitlab-ci-yml-coverage-key-is-unknown.yml

+---
+title: Document when current coverage configuration option was introduced
+merge_request: 9443
+author:

doc/ci/yaml/README.md

@@ -1003,6 +1003,9 @@ job:
 
 ### coverage
 
+**Notes:**
+- [Introduced][ce-7447] in GitLab 8.17.
+
 `coverage` allows you to configure how code coverage will be extracted from the
 job output.
 
@@ -1361,3 +1364,4 @@ CI with various languages.
 [ce-6669]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6669
 [variables]: ../variables/README.md
 [ce-7983]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7983
+[ce-7447]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7447