Docs for the allow_duplicates option added in elastic#61916

Author: danhermann

docs/reference/ingest/processors/append.asciidoc

@@ -17,6 +17,9 @@ Accepts a single value or an array of values.
 | Name      | Required  | Default  | Description
 | `field`  | yes       | -        | The field to be appended to. Supports <<accessing-template-fields,template snippets>>.
 | `value`  | yes       | -        | The value to be appended. Supports <<accessing-template-fields,template snippets>>.
+| `allow_duplicates` | no | true  | When false, ignores any of the values to be
+                                    appended that are already present in the
+                                    target field.
 include::common-options.asciidoc[]
 |======