resolve #261, explain args to callback to #del
resolve #261, explain args to callback to #del

Per bug report, example crafted from test.

djchie/levelup,lastguest/levelup,roppa/levelup,roppa/levelup,toddself/levelup,lastguest/levelup,Level/levelup,toddself/levelup,djchie/levelup,JTKnox91/levelup,timkuijsten/levelup,timkuijsten/levelup,JTKnox91/levelup