Update PR template for clarity (#8777)

Attempt to make the PR checklist more clear about expectations:
- Retitle the checklist to make it explicit that it's for review, not landing.
- Make it explicit that the version, CHANGELOG, and test steps require some action, rather than just personally believing that a PR should be exempt.

Author: stuartmorgan-g

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,18 +2,18 @@
 
 *List which issues are fixed by this PR. You must list at least one issue.*
 
-## Pre-launch Checklist
+## Pre-Review Checklist
 
 - [ ] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
 - [ ] I read the [Tree Hygiene] page, which explains my responsibilities.
 - [ ] I read and followed the [relevant style guides] and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use `dart format`.)
 - [ ] I signed the [CLA].
 - [ ] The title of the PR starts with the name of the package surrounded by square brackets, e.g. `[shared_preferences]`
 - [ ] I [linked to at least one issue that this PR fixes] in the description above.
-- [ ] I updated `pubspec.yaml` with an appropriate new version according to the [pub versioning philosophy], or this PR is [exempt from version changes].
-- [ ] I updated `CHANGELOG.md` to add a description of the change, [following repository CHANGELOG style], or this PR is [exempt from CHANGELOG changes].
-- [ ] I updated/added relevant documentation (doc comments with `///`).
-- [ ] I added new tests to check the change I am making, or this PR is [test-exempt].
+- [ ] I updated `pubspec.yaml` with an appropriate new version according to the [pub versioning philosophy], or I have commented below to indicate which [version change exemption] this PR falls under.
+- [ ] I updated `CHANGELOG.md` to add a description of the change, [following repository CHANGELOG style], or I have commented below to indicate which [CHANGELOG exemption] this PR falls under.
+- [ ] I updated/added any relevant documentation (doc comments with `///`).
+- [ ] I added new tests to check the change I am making, or I have commented below to indicate which [test exemption] this PR falls under.
 - [ ] All existing and new tests are passing.
 
 If you need help, consider asking for advice on the #hackers-new channel on [Discord].
@@ -26,7 +26,7 @@ If you need help, consider asking for advice on the #hackers-new channel on [Dis
 [Discord]: https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md
 [linked to at least one issue that this PR fixes]: https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#overview
 [pub versioning philosophy]: https://dart.dev/tools/pub/versioning
-[exempt from version changes]: https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#version
+[version change exemption]: https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#version
 [following repository CHANGELOG style]: https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#changelog-style
-[exempt from CHANGELOG changes]: https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#changelog
-[test-exempt]: https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#tests
+[CHANGELOG exemption]: https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#changelog
+[test exemption]: https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#tests