Problem: documentation missing for authority record identifier setting

**Documentation version**

* 2.7 and later

**Relevant pages**

* https://www.accesstomemory.org/docs/latest/user-manual/administer/settings/#identifier-settings

**Relevant sources and links**

* Redmine issue ticket: https://projects.artefactual.com/issues/13373
* Forum thread where this missing doc entry was found: https://groups.google.com/g/ica-atom-users/c/pAuUKSGvZtk/m/KRmijKOkAQAJ

**Description**

* A feature was added in 2.7 that when enabled, will prevent a user from saving an authority record that contains a duplicate identifier value. It appears that no documentation was added to the Settings page in the AtoM docs.

**Recommended fix**

* Add a new entry in the Identifiers section of the Settings docs
* Update the screenshot of all settings in the section included, so that it also includes the new setting

Open the Settings documentation at the linked Identifier Settings page and compare its Identifiers section and included all-settings screenshot with the authority-record identifier behavior described in the issue. Done means the new duplicate-identifier setting is documented and the screenshot includes it.

Written by the indexing model from the issue text.