Add docstring for Backend type class
Add docstring for Backend type class

Ignore-this: 1acb52e991be1ffa616ba439bab0d2d2

darcs-hash:20090807161841-e018d-2b366968219bbf3e51597f001f2152486218b348.gz

nick0x01/dbmigrations,nathankot/dbmigrations,jakubfijalkowski/dbmigrations,anton-dessiatov/dbmigrations,creswick/dbmigrations