Document Level.GetRoomByIdx

[Zamiell]

It should be noted that:

• You can provide any arbitrary number as an argument and get back a valid RoomDescriptor object, so error checking is recommended.
• The GridIndex property of an invalid RoomDescriptor object seems to always be -1. Note that this is not actually the RoomDescriptor of the Devil Room, as it will have a different VisitedCount.

[wofsauge]

Added infos in db0329f