4.4.0-72.93) is not available, but a newer one is (4.4.0-142.168). We redirected you there.
/usr/share/man/man3/XSetBackground.3.gz is in libx11-doc 2:1.6.2-1ubuntu2.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/man/man3/XSetState.3.gz