Those are the APIs of the BigWorld engine, which is closed-source and somewhat hard to obtain. However, this seems to feature documentation for an old version, which is, as far as I can say, mostly still accurate
Keep in mind, though, that BigWorld.player(), for example, returns an instance of PlayerAccount or PlayerAvatar in different contexts, which are defined inside wargaming's code, hence are present in the decompiled code. This is true for vehicles, arenas and lots of other things, although I can't be more precise since I don't have too much experience with all this