Referenceable: explain a bit more of the whys

hcl2template/addrs/referenceable.go

@@ -3,6 +3,8 @@ package addrs
 // Referenceable is an interface implemented by all address types that can
 // appear as references in configuration language expressions.
 type Referenceable interface {
+	// referenceableSigil is private to ensure that all Referenceables are
+	// implentented in this current package. For now this does nothing.
 	referenceableSigil()
 
 	// String produces a string representation of the address that could be
@@ -11,6 +13,8 @@ type Referenceable interface {
 	String() string
 }
 
+// referenceable is an empty struct that implements Referenceable, add it to
+// your Referenceable struct so that it can be recognized as such.
 type referenceable struct {
 }