settings_location.php - is there an example
The settings_location.php file should not normally be required, but an example is described at:

https://wiki.civicrm.org/confluence/pages/viewpage.action?pageId=83034188

The file civicrm.config.php should be able to handle bootstrap in most common configurations without settings_location.php. However, this depends on having somewhat plain/predictable directory structure, and it could break down when using things like (a) symlinks, (b) special filesystems, or (c) multi-tenant/multi-site installations.