Foundations Team: add another case where setting an Assignee makes sense.

wiki/src/contribute/working_together/roles/foundations_team.mdwn

@@ -190,6 +190,8 @@ these conditions is met:
  - Only one of us can complete the task. This helps identify
    bottlenecks, high bus factor, and over-commitment.
 
+ - This task is the assignee's current top priority wrt. Tails work.
+
  - Sponsor deliverables that are managed under the "let's decide
    a long time in advance who exactly will do each task" paradigm.