add flutter version constraint upper bound note (#5375)

Adding a note that the flutter version constraint is not fully enforced
-- see [this issue](flutter/flutter#95472)

---

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<details>
  <summary>Contribution guidelines:</summary><br>

- See our [contributor
guide](https://github.com/dart-lang/site-www/blob/main/CONTRIBUTING.md)
for general expectations for PRs.
- Larger or significant changes should be discussed in an issue before
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</details>

---------

Co-authored-by: Anthony Sansone <[email protected]>
Co-authored-by: Parker Lougheed <[email protected]>

src/tools/pub/pubspec.md

@@ -514,8 +514,15 @@ environment:
 
 A Flutter SDK constraint is satisfied only if pub is running in the
 context of the `flutter` executable, and the Flutter SDK's
-`version` file matches the given version constraint. Otherwise,
-the package will not be selected.
+`version` file meets the version constraint's lower bound. Otherwise,
+the package won't be selected.
+
+{{site.alert.note}}
+  Note: The Flutter SDK only enforces the lower bound of the flutter constraint.
+  To learn more, check out
+  [issue #95472](https://github.com/flutter/flutter/issues/95472)
+  in the `flutter/flutter` repository.
+{{site.alert.end}}
 
 To publish a package with a Flutter SDK constraint,
 you must specify a Dart SDK constraint with a minimum version of