/usr/include/ is in libdevhelp-dev 3.28.1-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/devhelp-3.0/
/usr/include/devhelp-3.0/devhelp/
/usr/include/devhelp-3.0/devhelp/devhelp.h
/usr/include/devhelp-3.0/devhelp/dh-assistant-view.h
/usr/include/devhelp-3.0/devhelp/dh-book-manager.h
/usr/include/devhelp-3.0/devhelp/dh-book-tree.h
/usr/include/devhelp-3.0/devhelp/dh-book.h
/usr/include/devhelp-3.0/devhelp/dh-completion.h
/usr/include/devhelp-3.0/devhelp/dh-enum-types.h
/usr/include/devhelp-3.0/devhelp/dh-init.h
/usr/include/devhelp-3.0/devhelp/dh-keyword-model.h
/usr/include/devhelp-3.0/devhelp/dh-link.h
/usr/include/devhelp-3.0/devhelp/dh-sidebar.h