It places the documentation at /usr/share/gtk-doc/html/libxml2/ Reproducible: Always
FEATURES="nodoc" is just to remove basic /usr/share/doc location docs (meant to be opened directly by users) rather than guess what every ebuilds may install and where that are often meant to be read by specific applications (e.g. qt-creator and assistant reads /usr/share/qt*-doc, and it would be an issue FEATURES="nodoc" casually deleted them, dev-qt/qt-docs would even install nothing). If you really want to remove other files and certain you don't need them, you can use portage's INSTALL_MASK variable directly (nodoc is a rough equivalent that only does INSTALL_MASK="/usr/share/doc").