Port viewport API documentation refinements.

[pengdev]

Summary of changes

This PR ports #1142 to v10.3 release branch.

This PR refines the public API documentation for viewport plugin, and align with iOS API docs.

User impact (optional)

Pull request checklist:

• Briefly describe the changes in this PR.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality. If tests were not written, please explain why.
• Optimize code for java consumption (@JvmOverloads, @file:JvmName, etc).
• Add example if relevant.
• Document any changes to public APIs.
• Apply changelog label ('breaking change', 'bug 🪲', 'build', 'docs', 'feature 🍏', 'performance ⚡', 'testing 💯') or use the label 'skip changelog'
• Add an entry inside this element for inclusion in the mapbox-maps-android changelog: <changelog></changelog>.
• If this PR is a v10.[version] release branch fix / enhancement, merge it to main firstly and then port to v10.[version] release branch.

Fixes: < Link to related issues that will be fixed by this pull request, if they exist >

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