Even though I have USE="-doc", numerous docbook stuff is being pulled in as dependencies of gnome-power-manager. I made the RDEPEND conditional so that this stuff didn't get merged and everything seems to work OK.
Created attachment 96358 [details, diff] patch I haven't actually looked into any potential reason why this might have been unconditional, I have only tested for negative effects of this change and haven't seen any.
The docbook-sgml-utils are needed for manpage generation.