[Don't merge yet] dev/core#161: presave hook documentation
Created by: omarabuhussein
Documenting a new hook added in this PR : https://github.com/civicrm/civicrm-core/pull/12253
Created by: omarabuhussein
Documenting a new hook added in this PR : https://github.com/civicrm/civicrm-core/pull/12253