Add changelog and doc

changelogs/unreleased/api-delete-respect-headers.yml

+---
+title: 'API: Respect the "If-Unmodified-Since" header when delting a resource'
+merge_request: 9621
+author: Robert Schilling
+type: added

doc/api/README.md

@@ -264,6 +264,7 @@ The following table shows the possible return codes for API requests.
 | `404 Not Found` | A resource could not be accessed, e.g., an ID for a resource could not be found. |
 | `405 Method Not Allowed` | The request is not supported. |
 | `409 Conflict` | A conflicting resource already exists, e.g., creating a project with a name that already exists. |
+| `412` | Indicates the request was denied. May happen if the `If-Unmodified-Since` header is provided when trying to delete a resource, which was modified in between. |
 | `422 Unprocessable` | The entity could not be processed. |
 | `500 Server Error` | While handling the request something went wrong server-side. |