Updates scroll cache extent doc (flutter#184142)

<!--
Thanks for filing a pull request!
Reviewers are typically assigned within a week of filing a request.
To learn more about code review, see our documentation on Tree Hygiene:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
-->

as title

## Pre-launch Checklist

- [ ] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [ ] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [ ] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [ ] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [ ] 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 `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

**Note**: The Flutter team is currently trialing the use of [Gemini Code
Assist for
GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code).
Comments from the `gemini-code-assist` bot should not be taken as
authoritative feedback from the Flutter team. If you find its comments
useful you can update your code accordingly, but if you are unsure or
disagree with the feedback, please feel free to wait for a Flutter team
member's review for guidance on which automated comments should be
addressed.

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[AI contribution guidelines]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#ai-contribution-guidelines
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: chunhtai

packages/flutter/lib/src/rendering/viewport.dart

@@ -23,7 +23,17 @@ import 'object.dart';
 import 'sliver.dart';
 import 'viewport_offset.dart';
 
-/// The amount of additional content to cache around the viewport.
+/// The amount of additional content to display and lay out around the viewport.
+///
+/// The cache area is used to lay out slivers before they are visible on screen.
+/// Items that fall in this cache area are laid out even though they are not
+/// visible on screen. This allows the viewport to render them ahead of time,
+/// enabling a smoother scrolling experience as items scroll into view.
+///
+/// This class encapsulates both the value and the style of the cache extent.
+/// It allows the cache extent to be defined either as a fixed number of logical
+/// pixels using [ScrollCacheExtent.pixels] or as a multiplier of the viewport's
+/// main axis extent using [ScrollCacheExtent.viewport].
 ///
 /// See also:
 ///
@@ -36,6 +46,12 @@ sealed class ScrollCacheExtent {
   const factory ScrollCacheExtent.pixels(double pixels) = _PixelScrollCacheExtent;
 
   /// Creates a cache extent as a multiplier of the viewport's main axis extent.
+  ///
+  /// The main axis extent is the size of the viewport in its main axis. For
+  /// example, for a vertically scrolling list, the main axis extent is the
+  /// height of the viewport. If the viewport is 600 logical pixels tall, then
+  /// `ScrollCacheExtent.viewport(2.0)` results in a cache extent of 1200 logical
+  /// pixels.
   const factory ScrollCacheExtent.viewport(double value) = _ViewportScrollCacheExtent;
 
   /// Returns the cache extent in logical pixels for a given [mainAxisExtent].