The official C++ “documentation” is the C++ standard, ISO/IEC 14882:2014(E). There is information at ISOCPP how to obtain the document.
I wouldn’t necessarily consider the standard good documentation but it does specify the behavior of the standard language and library constructs.
There isn’t any other official document on C++ describing the entire language. There are good derivative works making things more accessible like Bjarne Stroustrup’s “Programming: Principles as Practices Using C++” and Nicolai Josuttis’s “The C++ Standard Library”.