Mention SceneMultiplayer in high_level_multiplayer

[IfgiU]

This fixes #9717 by mentioning in the "High Level Multiplayer" Overview that the default implementation is a SceneMultiplayer. It also mentions that for object serializing allow_object_decoding is required.

[Jordyfel]

The change is technically incorrect, it could be a SceneMultiplayer (as it is by default), or it could be something else, that's why its a base class. The fact that it is the default implementation is mentioned clearly enough in MultiplayerAPI and SceneMultiplayer class reference.

That said, I agree that it would be good to have it mentioned here as well, but it should be worded better.