Enhance PR template with changelog and impact details (flutter#177333)

Updated the pull request template to include a changelog description
section and improved clarity on impact description.

This came up after multiple release engineering rotations where I needed
to ask full time maintinainters to rewrite their changelogs entries.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

Author: reidbaker

.github/PR_TEMPLATE/PULL_REQUEST_CP_TEMPLATE.md

@@ -4,18 +4,26 @@ Please fill in the form below, and a flutter domain expert will evaluate this ch
 ### Issue Link:
 What is the link to the issue this cherry-pick is addressing?
 
-< Replace with issue link here >
-
-### Changelog Description:
-Explain this cherry pick in one line that is accessible to most Flutter developers. See [best practices](https://github.com/flutter/flutter/blob/main/docs/releases/Hotfix-Documentation-Best-Practices.md) for examples
-
-< Replace with changelog description here >
+https://github.com/flutter/flutter/issues/< Replace with issue link here >
 
 ### Impact Description:
-What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)? Does it impact development (ex. flutter doctor crashes when Android Studio is installed), or the shipping production app (the app crashes on launch)
+What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)?
+Does it impact development (ex. flutter doctor crashes when Android Studio is installed),
+or the shipping of production apps (the app crashes on launch).
+This information is for domain experts and release engineers to understand the consequences of saying yes or no to the cherry pick.
 
 < Replace with impact description here >
 
+### Changelog Description:
+Explain this cherry pick:
+* In one line that is accessible to most Flutter developers.
+* That describes the state prior to the fix.
+* That includes which platforms are impacted.
+See [best practices](https://github.com/flutter/flutter/blob/main/docs/releases/Hotfix-Documentation-Best-Practices.md) for examples.
+
+< Replace with changelog description here >
+[flutter/<ISSUE_NUMBER>] When <SCENARIO> [on <PLAFORMS(S)>], <DESCRIPTION>
+
 ### Workaround:
 Is there a workaround for this issue?