Document label priorities

• https://gitlab.com/gitlab-org/gitlab-ce/issues/14189
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4009

Document:

1. usability (how to prioritize, click star, prioritized labels only on 1st page, drag and drop)
2. behavior (how label priority is assigned, explain null)
3. usage in the issue tracker (using the priority filter)

Anything else I missed?

cc @rymai @smcgivern