Move LuaControl / LuaPlayer / Character api note to json field like subclasses

[sparr]

Many fields and methods of LuaControl have "When called on a LuaPlayer, it must be associated with a character (see LuaPlayer::character)." literally contained in their description.

I am requesting that this instead be represented by a field in the json, similar to how the subclasses field is used to generate "Can only be used if this is ..." in the docs description.