Update amulet docs to reflect new UnitSentry access

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
Add docs for d.unrelate()

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
Merge pull request #196 from tvansteenburgh/amulet-1.6.0

Amulet 1.6.0 - Doc updates for new features