> Is C++ the most heavily documented programming language?

I doubt it. C++ does have a huge amount of stuff written by Bjarne - its author - because it appears C++ is the only thing he ever did† and he sure does like writing so that's what all his writing is about. He's written several books about C++, various HOPL papers about C++, as well as numerous proposal papers, speeches, standards documents, letters, all about C++

C++ is also enormous and undisciplined, so while there are a lot of C++ documents there's a lot more of C++ to be documented than most languages and I suspect that out-weighs the volume of documents.

† His PhD thesis is, at least for now, only available for personal inspection and I don't care enough to travel to the holding library and read it, but it seems plausible given the topic and timing that the work for that PhD thesis is also just C++ again, or rather a distant ancestor of C++