[image_picker] Clarify maxDuration parameter behavior

[777genius]

Description

Updates documentation to explicitly state that the maxDuration parameter only applies when recording videos from the camera (ImageSource.camera), and is ignored when selecting videos from the gallery (ImageSource.gallery).

This addresses confusion reported in issue #83630 where developers expected maxDuration to filter gallery video selections, but it only controls camera recording duration.

Changes

• Updated README.md with detailed explanation and code examples showing the difference between camera recording and gallery selection
• Enhanced dartdoc comments for pickVideo() method with important notes about gallery behavior
• Added clear warnings that maxDuration is ignored for gallery selections

Related Issues

Fixes https://github.com/flutter/flutter/issues/83630

Pre-Review Checklist

• [ ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ ] I read the Tree Hygiene page, which explains my responsibilities.
• [ ] I read and followed the relevant style guides and ran the auto-formatter.
• [ ] I signed the CLA.
• [ ] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [ ] I linked to at least one issue that this PR fixes in the description above.
• [ ] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under[^1].
• [ ] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under[^1].
• [ ] I updated/added any relevant documentation (doc comments with ///).
• [ ] I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under[^1].
• [ ] All existing and new tests are passing.

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