Add documentation for usage of encrypted postgresql passwords.

library/database/postgresql_user

@@ -44,6 +44,7 @@ options:
   password:
     description:
       - set the user's password, before 1.4 this was required.
+      - "When passing an encrypted password it must be generated with the format C('str(\"md5\") + md5( password + username )'), resulting in a total of 35 characters.  An easy way to do this is: C(echo \"md5$(echo -n \"verysecretpasswordJOE\" | md5)\")."
     required: false
     default: null
   db: