Add documentation for Db interface.

db.go

@@ -37,6 +37,10 @@ const (
 	InsertValidatedInput
 )
 
+// Db defines a generic interface that is used to request and insert data into
+// the bitcoin block chain.  This interface is intended to be agnostic to actual
+// mechanism used for backend data storage.  The AddDBDriver function can be
+// used to add a new backend data storage method.
 type Db interface {
 	// Close cleanly shuts down the database and syncs all data.
 	Close()