[doc] no hint on how to use db_version

[Christophe Combelles]

Trying to use db_version, the documentation doesn't tell how to use it, particularly how to set it.
(it seems that there is a settable property that write the db)

[Christophe Combelles (ccomb)]

the doc should also say that the upgrade script should end with a commit()

[Georges Racinet (gracinet)]

yes it's simply a settable property, but the obvious still needs to be documented somehow, you're right.

[Georges Racinet (gracinet)]

db_version setting is automatic from VERSIONS.txt file
cr.commit() is also automatic (in case of success only)

Just updated doc to state this explicitely : http://docs.anybox.fr/anybox.recipe.openerp/stable/scripts.html#upgrade-scripts, after the example and in the "about versions" note