emerge -avutDN world [ebuild U ] dev-libs/glib-2.24.1-r1 [2.22.5] USE="fam hardened xattr -debug -doc (-selinux)" 0 kB [ebuild N ] dev-util/gtk-doc-am-1.15 0 kB [ebuild N ] app-text/docbook-xml-dtd-4.3-r1 0 kB [ebuild N ] dev-libs/libxslt-1.1.26 USE="crypt python -debug" 0 kB [nomerge ] app-text/docbook-xml-dtd-4.3-r1 [ebuild N ] app-text/sgml-common-0.6.3-r5 0 kB [ebuild N ] app-text/docbook-xsl-stylesheets-1.75.2 0 kB [ebuild N ] app-text/build-docbook-catalog-1.4 0 kB glib should not depend on: dev-util/gtk-doc-am if useflag doc isn't set.
Created attachment 241103 [details, diff] fixes ebuild to not include dev-util/gtk-doc-am if USE="-doc" is set.
you've misunderstood what gtk-doc-am is about, it is needed for eautoreconf and one package that is less that a hundred kbytes without any compilation won't kill you.
*** Bug 330747 has been marked as a duplicate of this bug. ***
In my opinion -doc should not install docbook-xml.