Fix d.unrelate()

Fixes: https://bugs.launchpad.net/amulet/+bug/1321305

Docs are staged on a branch pending the amulet-1.6.0 release: https://github.com/tvansteenburgh/docs/compare/amulet-1.6.0

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>