docs(splunk): document splunk-index-acknowledgment logging driver option

content/manuals/engine/logging/drivers/splunk.md

@@ -74,6 +74,7 @@ The following properties let you configure the Splunk logging driver.
 | `splunk-verify-connection`  | optional | Verify on start, that Docker can connect to Splunk server. Defaults to true.                                                                                                                                                                                                                                                               |
 | `splunk-gzip`               | optional | Enable/disable gzip compression to send events to Splunk Enterprise or Splunk Cloud instance. Defaults to false.                                                                                                                                                                                                                           |
 | `splunk-gzip-level`         | optional | Set compression level for gzip. Valid values are -1 (default), 0 (no compression), 1 (best speed) ... 9 (best compression). Defaults to [DefaultCompression](https://golang.org/pkg/compress/gzip/#DefaultCompression).                                                                                                                    |
+| `splunk-index-acknowledgment` | optional | Set to `true` if index acknowledgment is enabled for the given index on the HTTP Event Collector. When enabled, the driver sends the `X-Splunk-Request-Channel` header with each event, which is required for acknowledged writes. Defaults to `false`. |
 | `tag`                       | optional | Specify tag for message, which interpret some markup. Default value is `{{.ID}}` (12 characters of the container ID). Refer to the [log tag option documentation](log_tags.md) for customizing the log tag format.                                                                                                                         |
 | `labels`                    | optional | Comma-separated list of keys of labels, which should be included in message, if these labels are specified for container.                                                                                                                                                                                                                  |
 | `labels-regex`              | optional | Similar to and compatible with `labels`. A regular expression to match logging-related labels. Used for advanced [log tag options](log_tags.md).                                                                                                                                                                                           |