mejorar docs Multiplayer

/docs/MULTIPLAYER.md

- Detallar la dinámica de juego
- Incluir capturas de pantalla (cuando sea posible)
- Formatear texto
- Detalle endpoints
- Explicitar seteo en admin

Start by reading /docs/MULTIPLAYER.md and inspect the project’s multiplayer entry points, admin setup, and endpoint definitions to verify the current behavior. Update the document with the game flow, endpoint details, admin configuration, improved formatting, and screenshots where available; done means each requested topic is covered accurately.

Written by the indexing model from the issue text.