install ubuntu c++ manpage

 Used to simply check the contents of c++ std api in the terminal

No manpage

$ man std::lock_guard                        
No manual entry for std::lock_guard

Install manpage

$ sudo apt install libstdc++-10-doc

Show manpage

$ man std::lock_guard | cat
std::lock_guard< _Mutex >(3cxx)                                                                                                                                                                                                       std::lock_guard< _Mutex >(3cxx)

NAME
       std::lock_guard< _Mutex > - A simple scoped lock type.
......