Add JSDoc comment to mutation return value behavior

[kobiebotha]

Builds off powersync-ja/powersync-docs#378 (which was inspired by #865 (comment))

When using the default client-side JSON-based view system, rowsAffected on QueryResult may be 0 for successful UPDATE and DELETE statements.

This PR documents the behavior and recommends using a RETURNING clause if needed.

Not including a changeset since the docs can update when the next batch of changes are released.

[changeset-bot]

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Latest commit: 2844b1d

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