improve docs for icc_*
improve docs for icc_*

note use of _get_typeof() to spot embedded profiles, see

https://github.com/jcupitt/libvips/issues/511

lovell/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips