update GitLab policy to reflect labels in use

We add the Next label because it's already being used in a subproject
of tpa (gitlab) and because it's used globally. It also helps us
separate the "endless pile of ticket" in Open from the Backlog, as
now we can throw more things in the backlog without worrying about
overloading our monthly capacity.

policy/tpa-rfc-5-gitlab.md

@@ -53,15 +53,26 @@ felt we do not really issue "releases" of our software, since we have
 too many moving parts to cohesively release those as a whole.
 
 Instead, it is suggested we adopt the [Kanban][] development strategy
-which is implemented in GitLab as [issue boards](https://docs.gitlab.com/ee/user/project/issue_board.html). Issues would
-first land into a "triage" queue, then get assigned to a specific
-milestone as the ticket gets planned. The default `To do` and `Doing`
-boards should be fine to get started, although we could go with
-per-month labels as well. That could be adjusted in the future without
-formally reviewing this policy.
-
-Priority of items in the `To do` queue are determined by the *order*
-of items in the stack. Tickets should *not* stay in the `To do` queue
+which is implemented in GitLab as [issue boards](https://docs.gitlab.com/ee/user/project/issue_board.html). 
+
+Issues first land into a "triage" queue (`Open`), then get assigned to
+a specific milestone as the ticket gets planned. We use the `Backlog`,
+`Next`, and `Doing` of the global "TPO" group board labels. With the
+`Open` and `Closed` list, this gives us the following policy:
+
+ * `Open`: untriaged ticket, "ice box"
+ * `Backlog`: planned work
+ * `Next`: work to be done in the next iteration or "sprint"
+   (e.g. currently a month)
+ * `Doing`: work being done right now (generally during the day or
+   week)
+ * `Closed`: completed work
+
+That list can be adjusted in the future without formally reviewing
+this policy.
+
+Priority of items in the lists are determined by the *order* of items
+in the stack. Tickets should *not* stay in the `Next` or `Doing` lists
 forever and should instead actively be closed or moved back into the
 `Open` or `Backlog` board.