4.15.0-24.26) is not available, but a newer one is (4.15.0-46.49). We redirected you there.
/usr/share/man/man3/XUniqueContext.3.gz is in libx11-doc 2:1.6.2-1ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/man/man3/XSaveContext.3.gz