Document overlay sizing constraints

[mvincentong]

Fixes #48052.

Issue: The reported behavior comes from Overlay layout, not from CompositedTransformFollower: non-positioned overlay entries are laid out with tight constraints matching the overlay's size, so a fixed-size visual child needs to be positioned or sized inside the entry.

Fix:

• Moves the guidance to the Overlay class docs.
• Documents how Overlay chooses its own size for finite constraints, infinite constraints, and OverlayEntry.canSizeOverlay.
• Documents that non-positioned overlay entries receive tight overlay-sized constraints, and points readers to sizing/positioning widgets such as Positioned and Align.

Tests:

• ./bin/dart format --output=none --set-exit-if-changed packages/flutter/lib/src/widgets/basic.dart packages/flutter/lib/src/widgets/overlay.dart
• ./bin/flutter analyze packages/flutter/lib/src/widgets/basic.dart packages/flutter/lib/src/widgets/overlay.dart
• git diff --check

Risk: Documentation-only. This updates the location and wording of the guidance to match the underlying layout behavior.

[gemini-code-assist]

Code Review

This pull request adds documentation to clarify how CompositedTransformFollower handles layout constraints and the compositing phase. Feedback suggests improving technical precision regarding the timing of the transform, using brackets for class links per the style guide, and adjusting the recommended wrapping strategy for better hit testing.

[mvincentong]

Updated the docs wording in head 42bb1bb to make the layout/compositing distinction more precise, link Overlay, and recommend loosening constraints around the follower itself. Reran format, targeted analyze, and git diff --check.

[QuncCccccc]

LGTM. The doc makes sense to me, but I'm not familiar with CompositedTransformFollower so would like to wait @chunhtai or @victorsanni for another round of review:)

Thanks for your contribution!

[LongCatIsLooong]

I looked at #48052 and it doesn't seem to have to do with CompositedTransformFollower, it's just misunderstanding of how Constraints work. I don't think this the right place for the documentation, since it's the parent that's enforcing the size.

[LongCatIsLooong]

I think we should probably add documentation to the Overaly class to explain how it sizes itself (also the class documentation does not seem to mention size-determining child) and what kind of constraints it gives to non-Positioned children.

[mvincentong]

Moved this guidance to Overlay in 9c3a503, matching the review feedback that the issue is Overlay sizing/constraints rather than CompositedTransformFollower behavior. The PR now documents Overlay sizing, canSizeOverlay, and the tight constraints used for non-positioned overlay entries. Verification rerun: formatter, targeted analyze, and git diff --check pass.