Document the argument count limit in `SceneTree.call_group()`
Document the argument count limit in `SceneTree.call_group()`

This limitation should be lifted in 4.0 thanks to the new method
binding system, but it's still present in 3.2.x.

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot