Documentation for master branch can be confused for shipped versions

In https://gitlab.com/gitlab-org/gitlab-ce/issues/58357#note_146855908, we saw a feature that we are going to ship in GitLab 11.9 be mistaken for something already available. Couple of questions:

1. Should the latest official release (e.g. 11.8) be the default rather than master?

2. Can the version number of the release be called out in a more obvious way? The master at the top doesn't give any warnings, such as NOTE: This documentation reflects the latest pre-release version of GitLab. Be sure to select the correct version.