As it is, there is no way to install docs for Qt without recompiling the entire library.
Since this is a delicate operation, especially for KDE users, it would be much better to have a separate "qt-doc" ebuild to allow people to install the docs at a later stage without recompiling.
Note also that, without the documentation, tools like QtAssistant and QtDesigner, deeply integrated with the Qt docs, are basically broken, but the user is not informed when compiling these ebuilds.
Well, that simply won't work, see Bug 46798 e.g. It was already there and was removed exactly because of this.