Adds note about color equality after wide gamut. (#11132)

## _Description of what this PR is changing or adding, and why:_
Adding more details about the changing of Color equality.

## _Issues fixed by this PR (if any):_
flutter/flutter#127855

## _PRs or commits this PR depends on (if any):_
flutter/engine#54981

## Presubmit checklist

- [x] This PR is marked as draft with an explanation if not meant to
land until a future stable release.
- [x] This PR doesn’t contain automatically generated corrections
(Grammarly or similar).
- [x] This PR follows the [Google Developer Documentation Style
Guidelines](https://developers.google.com/style) — for example, it
doesn’t use _i.e._ or _e.g._, and it avoids _I_ and _we_ (first person).
- [x] This PR uses [semantic line
breaks](https://github.com/dart-lang/site-shared/blob/main/doc/writing-for-dart-and-flutter-websites.md#semantic-line-breaks)
of 80 characters or fewer.

---------

Co-authored-by: Shams Zakhour (ignore Sfshaza) <[email protected]>

Author: gaaclarke

src/content/release/breaking-changes/wide-gamut-framework.md

@@ -141,6 +141,21 @@ final x = color.withOpacity(0.0);
 final x = color.withValues(alpha: 0.0);
 ```
 
+### Equality
+
+Once `Color` stores its color components as floating-point numbers,
+equality works slightly differently.
+When calculating colors there might be tiny
+difference in values that could be considered equal.
+To accommodate this use the [`closeTo`][] matcher or the [`isColorSameAs`][] matcher.
+
+```dart
+// Before
+expect(calculateColor(), const Color(0xffff00ff));
+// After
+expect(calculateColor(), isSameColorAs(const Color(0xffff00ff)));
+```
+
 ## Timeline
 
 ### Phase 1 - New API introduction, old API deprecation
@@ -168,3 +183,5 @@ Relevant PRs:
 [Impeller]: {{site.api}}/perf/impeller
 [wide gamut color spaces]: https://en.wikipedia.org/wiki/RGB_color_spaces
 [inheritance to composition]: https://en.wikipedia.org/wiki/Composition_over_inheritance
+[`closeTo`]: {{site.api}}/documentation/matcher/latest/matcher/closeTo.html
+[`isColorSameAs`]: {{site.api}}/flutter/flutter_test/isSameColorAs.html