docs(messaging): Fix documentation for localized messages

[TarekkMA]

Description

Refrences: https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages https://developer.apple.com/documentation/usernotifications/generating-a-remote-notification#Localize-your-alert-messages

Related Issues

Closes #12691

Checklist

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• [ ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ ] My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• [ ] All existing and new tests are passing.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [ ] The analyzer (melos run analyze) does not report any problems on my PR.
• [ ] I read and followed the Flutter Style Guide.
• [ ] I signed the CLA.
• [ ] I am willing to follow-up on review comments in a timely manner.

Breaking Change

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• [ ] Yes, this is a breaking change.
• [ ] No, this is not a breaking change.