Updated clarification for regression priority per the latest label organization

This is the counterpart documentation update to the process change in : https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20623

Important: For RMs, if this release is 11.2 it means that only bugs with regression:11.2 can be picked into the stable release branch after the 7th of next month. The regression regression label alone will no longer suffice. Anything that is not a regression that affects 11.2 goes into the same queue with the rest of the workload.