[notification] fix documentation
The doc say precommit, the code says postcommit. The code is right.
We also clarify the documentation of another related operation looking suspiciously
wrong but actually correct.
change_attribute_type('CWCache', 'timestamp', 'TZDatetime')
change_attribute_type('CWUser', 'last_login_time', 'TZDatetime')
change_attribute_type('CWSource', 'latest_retrieval', 'TZDatetime')
drop_attribute('CWSource', 'synchronizing')
add_attribute('CWSource', 'in_synchronization')