Inform about event management behavior change in 9.10

Since 9.10, when enabling the "save" option in one of the following operations

• Document.Update
• DocumentMultivaluedProperty.addItem
• Document.SetProperty
• Document.RemoveProperty
• Document.AddItemToListProperty

in the context of the "empty document created", "about to create" events, Nuxeo Server will throw an exception (it used to ignore the option).

TODO
Inform about this behavior change

• In upgrade notes https://doc.nuxeo.com/910/nxdoc/upgrade-from-lts-2016-to-lts-2017/ (list all operations impacted listed in NXP attached)
• In Studio doc specific upgrades page
• Maybe in other pages where it makes sense