As I understand it depending on the platform they use one of three compilers, GCC, Clang, MSVC. Over time the versions used have changed. The language is still evolving and there will be more changes as time passes. The best I can suggest is look at the compiler output and determine which one is being used and what version. You can then go on line and find the documentations for that particular version.
It is also important to note that he 'Arduino language' is C++, but it is very different from most C++ varieties. The 'Arduino language' has a lot of abstraction built in to it, especially involving the hardware interfaces. These abstractions have made it very simple to use. This was designed in Italy as a learning device for students. Here is a good read: Invention Story and History of Development of Arduino