Install documentation for GCC on Debian Buster

If you source.list only contains the following standard sources for Debian Buster, there will be no man pages for your GCC compilers.

deb http://mirrors.163.com/debian/ buster main contrib non-free

deb-src http://mirrors.163.com/debian/ buster main contrib non-free

deb http://security.debian.org/debian-security/ buster/updates main contrib

deb-src http://security.debian.org/debian-security/ buster/updates main contrib

deb http://mirrors.163.com/debian/ buster-updates main contrib non-free

deb-src http://mirrors.163.com/debian/ buster-updates main contrib non-free

To install the documentation for GCC, you need to add backports to your source.list as below, where the contrib category contains the package gcc-doc and the non-free category contains the package gcc-8-doc.

deb http://mirrors.163.com/debian/ buster-backports main contrib non-free

The meaning of backports can be referred to here.

Backports are recompiled packages from testing (mostly) and unstable (in a few cases only, e.g. security updates) in a stable environment so that they will run without new libraries (whenever it is possible) on a Debian stable distribution.

Backports cannot be tested as extensively as Debian stable, and backports are provided on an as-is basis, with risk of incompatibilities with other components in Debian stable. Use with care!