Improved documentation for SpringSimulation

[drown0315]

Add a code snippet demonstrating the usage of SpringSimulation in the docs. Improve the descriptions of each argument in SpringDescription.

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.
• [ ] 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.

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[drown0315]

@justinmc Thanks for review, I've already made overall updates.

[drown0315]

@justinmc Thanks for review👍, I've already made overall updates.

[auto-submit[bot]]

auto label is removed for flutter/flutter/146674, due to This PR has not met approval requirements for merging. The PR author is not a member of flutter-hackers and needs 1 more review(s) in order to merge this PR.

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