update pr section

Author: jenshu

.github/ISSUE_TEMPLATE/enhancement.md

@@ -11,17 +11,18 @@ assignees:
 - One-line enhancement description (can be used as a release note):
 - Kubernetes Enhancement Proposal: <!-- link to kubernetes/enhancements file; if none yet, link to PR -->
 - Discussion Link: <!-- link to SIG mailing list thread, meeting, or recording where the Enhancement was discussed before KEP creation -->
-- [ ] Alpha
-  - [ ] KEP (`k/enhancements`) update PR(s):
-  - [ ] Code (`k/k`) update PR(s):
-  - [ ] Docs (`k/website`) update PR(s):
+- PRs by stage and milestone:
+  - [ ] Alpha - v1.xx
+    - [ ] KEP (`k/enhancements`) update PR(s):
+    - [ ] Code (`k/k`) update PR(s):
+    - [ ] Docs (`k/website`) update PR(s):
 
 <!-- Uncomment these as you prepare the enhancement for the next stage
-- [ ] Beta
+- [ ] Beta - v1.xx
   - [ ] KEP (`k/enhancements`) update PR(s):
   - [ ] Code (`k/k`) update PR(s):
   - [ ] Docs (`k/website`) update(s):
-- [ ] Stable
+- [ ] Stable - v1.xx
   - [ ] KEP (`k/enhancements`) update PR(s):
   - [ ] Code (`k/k`) update PR(s):
   - [ ] Docs (`k/website`) update(s):