This package installs HTML documentation in /usr/share/gtk-doc/ even when it is emerged with USE="-doc". Since I do not have dev-util/gtk-doc installed, I guess the doc files have been pre-generated by upstream and included in the tarball, from where they are installed by default.
unfortunately you got it wrong, USE="-doc" in gnome related packages means you don't rebuild docs, not that you don't install docs. You could come and discuss this on #gentoo-desktop, it would have spared you from filling those bugs.