Update regression labeling

PROCESS.md

@@ -255,6 +255,7 @@ When a regression is found:
   - If the regression is ~S1 or ~S2, set the current milestone
     - If the regression was introduced in a RC of the current release, label with ~Deliverable 
     - If the regression was introduced in the previos release, label with ~"Next Patch Release"
+  - If the regression is ~S3 or ~S4, it can be scheduled for later milestones
 
 When a new issue is found, the fix should start as soon as possible. You can
 ping the Engineering Manager or the Product Manager for the relative area to