docs: using multiple `extends` in gitlab-ci.yml introduced in GitLab 12

https://docs.gitlab.com/ee/ci/yaml/#extends Introduced in GitLab 11.3.

but

It’s also possible to use multiple parents for extends.

seems to have been introduced in GitLab 12, see milestone in https://gitlab.com/gitlab-org/gitlab-ce/issues/53134

Here is the example from the docs in GitLab-CI-Lint in version gitlab-ee:11.11.2-ee.0: