Man pages matching name std::__shared_mutex_cv in section 3cxx:

std::__shared_mutex_cv.3cxx.en	debian 10 / libstdc++-7-doc
std::__shared_mutex_cv.3cxx.en	debian 10 / libstdc++-8-doc
std::__shared_mutex_cv.3cxx.en	debian 11 / libstdc++-9-doc