docs: Add guide for multiple pending transactions

[KyleAMathews]

🎯 Changes

Added documentation explaining how multiple pending transactions work in the mutations guide. This section clarifies the layering behavior when users make rapid sequential mutations to the same item before earlier ones have synced back from the server.

The new section includes:

• An explanation of how transactions are layered with the most recent taking priority
• A concrete code example showing rapid updates (A → B → C)
• A detailed walkthrough of how the optimistic state is recomputed as each transaction syncs back
• Context on why this behavior provides a good user experience for rapid sequential updates

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm test:pr.

🚀 Release Impact

• This change is docs/CI/dev-only (no release).

https://claude.ai/code/session_01HruHi63EH2HhdQy86tWteX

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Latest commit: 1bd5cc1

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commit: 1bd5cc1

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