[shared_preferences] Add missing DartDoc

[mklim]

Description

Also adds a lint to prevent further undocumented methods.

Related Issues

Fixes flutter/flutter#44403.

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).
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• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
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[jmagman]

LGTM