Provide in-context explanation for issue tags

Issue tags are used for a wide variety of purposes, but some have a specific meeting that helps project maintainers or core committers keep track of key issues. To make it easier to understand what issue tags mean, we'll be providing some in-context explanation.

You can help to maintain the issue tags here: https://www.drupal.org/drupalorg/docs/maintenance/maintaining-issue-tags

Production outage: No
Change affects: Drupal.org issue queues
Deployment schedule: week of 2017-01-02
Backout plan: deployed