docs: correct v10 render priority migration guidance

[ICOM725]

Summary

• Clarify that numeric useFrame priority > 0 still disables default rendering in v10 for backwards compatibility, with a deprecation warning, and recommend { phase: 'render' }.
• Remove the incorrect double-rendering warning and use non-positive priorities in the v9 ordering-only example.

Fixes #3793.

Verification

• pnpm exec vitest run packages/fiber/tests/useFrame.test.tsx packages/fiber/tests/scheduler-integration.test.tsx (34 passed).
• Parsed the MDX page with Prettier, checked the changed section's formatting explicitly, and parsed both examples in that section with TypeScript.
• Checked the wording against the current useFrame implementation, default-render guard, and frame-loop guide.
• git diff --check.

Documentation only; runtime code is unchanged. Full CI and the hosted documentation build were not run.