[iPhone Duo] update uiscenedelegate documentation for scene geometry change - #14012

#14014)

…eometry change (#14012)"

This reverts commit 626cfa2.

Sorry I had to revert it until the change is landed to stable. 

_Description of what this PR is changing or adding, and why:_

_Issues fixed by this PR (if any):_

_PRs or commits this PR depends on (if any):_

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This is a follow up to flutter#195199

As discussed [in this
thread](flutter#195199 (comment)),
For a fixed-size FlutterView, `viewDidLayoutSubviews` is called when
switching from outer to inner display, but not called when switching
from inner to outer display. This is a puzzling behavior (possibly a bug
in UIKit), but it does prove that relying on `viewDidLayoutSubviews` is
not complete.

The only API we can find that triggers when a scene changes screen is
`didUpdateEffectiveGeometry`. There won't be a breaking change, but it
would require `FlutterSceneLifeCycleProvider` users to add a one-liner
for this to work. We should update our website to include this
([PR](flutter/website#14012)):

```
@available(iOS 26.0, *)
func windowScene(_ windowScene: UIWindowScene,
                 didUpdateEffectiveGeometry previousEffectiveGeometry: UIWindowScene.Geometry) {
  sceneLifeCycleDelegate.windowScene(windowScene, didUpdateEffectiveGeometry: previousEffectiveGeometry)
}
```

## Testing

I have manually verified with a fixed-size 300x300 FlutterView, and
confirmed the information is correctly updated when switching screen.

It is not possible to write integration test - I don't think there's a
`simctl` sub-command that allows us to fold/unfold the simulator. We
could add a integration test in the future if such api become available.

Fixes flutter#195177

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