*: add/fix godoc comments to satisfy golint

Fixes things like:
 * exported type/method/function X should have comment or be unexported
 * comment on exported type/method/function X should be of the form "X ..."
   (with optional leading article)

Refs. #213.

pkg/interop/iterator/iterator.go

@@ -11,8 +11,8 @@ func Create(items []interface{}) Iterator {
 	return Iterator{}
 }
 
-// TODO: Better description for this.
 // Key returns the iterator key.
+// TODO: Better description for this.
 func Key(it Iterator) interface{} {
 	return nil
 }