[collection] Clarify map modification during iteration

[khanak0509]

This PR updates the Map and HashMap documentation to explicitly state that updating the value of an existing key is allowed during iteration.

PR #62175

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[khanak0509]

Hii , I also checked map.dart, should I also update the class-level notice there too.

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https://dart-review.googlesource.com/c/sdk/+/491620 has been updated with the latest commits from this pull request.

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https://dart-review.googlesource.com/c/sdk/+/491620 has been updated with the latest commits from this pull request.

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https://dart-review.googlesource.com/c/sdk/+/491620 has been updated with the latest commits from this pull request.

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