Fix documentation in MultiplayerAPIExtension.xml
In implementing a MultiplayerAPIExtension, I found that the example from documentation did not function even in the Multiplayer tutorial example.
Eventually, I found that the issue was that base_multiplayer will not poll unless told to by the LogMultiplayer implementation.
I also corrected a spelling issue and a few out-of-date return types.
We don't use that info for anything, and it generates unnecessary diffs
every time we bump the minor version (and CI failures if we forget to
sync some files from opt-in modules (mono, text_server_fb).
- RPC configurations are now dictionaries.
- Script.get_rpc_methods renamed to Script.get_rpc_config.
- Node.rpc[_id] and Callable.rpc now return an Error.
- Refactor MultiplayerAPI to allow extension.
- New MultiplayerAPI.rpc method with Array argument (for scripts).
- Move the default MultiplayerAPI implementation to a module.
