/usr/share/man/man3/XmbufCreateBuffers.3.gz is in libxext-dev 2:1.3.3-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/man/man3/Xmbuf.3.gz
/usr/share/man/man3/Xmbuf.3.gz