Please clarify status of matrix builds/tests

I am looking for definitive information on Travis style matrix build support in GitLab CI. If this is supported, the documentation is missing entirely.

I was about to give up but Issue gitlab-org/gitlab-ci#87 suggests that matrices are supported ([since] introduction of build matrix). gitlab-org/gitlab-ci!109 was closed with a comment we chose another strategy [of matrix builds] with .gitlab-ci.yml But that was all I could track down.

If matrix builds are supported, please document the option. If they are not, please consider adding this feature; if this is the case, it would have been helpful to explicitly mention in the documentation that the outer-product build/test option does not exist, since the feature, in one form or another, is pretty much expected by default from a modern CI system.