docs: clarify computed return type checking

[KesleyDavid]

Description of Problem

The computed() typing guide shows a generic argument, but generic inference does not perform excess property checks on an object literal. Readers who need that check have no documented alternative.

Closes #3286

Proposed Solution

Add a concise return type annotation example to the existing computed() typing section. It shows the TypeScript error for an extra object property while keeping the generic example intact.

Additional Information

• Impact: readers can choose a return type annotation when they need stricter object-literal checks.
• Root cause: this is TypeScript generic inference behavior, not a Vue runtime issue.
• Validation: pnpm run build
• AI assistance note: This small documentation clarification was drafted with AI assistance and reviewed by the contributor.

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