Fix CarouselView demo page is not functional using mouse scroll (flutter#166512)

### Description
This PR seeks to refine the CarouselView API doc sample for Web and
Desktop platforms. On these platforms, mouse dragging gestures do not
provide natural behavior similar to other desktop applications. This PR
will improve the documentation by suggesting appropriate approaches to
users (either using Shift + mouse wheel to scroll the view horizontally
or referring to the instructions for overriding the default
ScrollBehavior if they still want to implement dragging on these
platforms).


### Related issue
Fix flutter#165265

## 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].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] 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.

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

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[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: huycozy

packages/flutter/lib/src/material/carousel.dart

@@ -107,6 +107,15 @@ import 'theme.dart';
 /// Here is an example to show different carousel layouts that [CarouselView]
 /// and [CarouselView.weighted] can build.
 ///
+/// On desktop and web running on desktop platforms, dragging to scroll with a mouse
+/// is disabled by default to align with natural behavior.
+///
+/// To further align expected behavior like this, mouse input can scroll horizontally
+/// by pressing the shift key while scrolling with the mouse wheel.
+///
+/// This key-driven behavior is dictated by the [ScrollBehavior.pointerAxisModifiers],
+/// while [ScrollBehavior.dragDevices] manages what devices can drag a scrollable.
+///
 /// ** See code in examples/api/lib/material/carousel/carousel.0.dart **
 /// {@end-tool}
 ///