Signal server_disconnected is emitted on the server

**Your Godot version:**
4.7.2.stable

**Issue description:**
The documentation for the `server_disconnected` signal in MultiplayerAPI explicitly states that it is only emitted on clients. This is not true: it is also emitted on the server when the server closes its own multiplayer peer.

See for yourself with this minimal reproduction project: [mrp-server-disconnected.zip](https://github.com/user-attachments/files/31813694/mrp-server-disconnected.zip)

**URL to the documentation page:**
[Here](https://docs.godotengine.org/en/stable/classes/class_multiplayerapi.html#class-multiplayerapi-signal-server-disconnected)

Start at the MultiplayerAPI documentation page linked in the issue and review the server_disconnected signal description. Use the attached minimal reproduction project to confirm the server behavior, then update the documentation so it accurately states when the signal is emitted.

Written by the indexing model from the issue text.