Fix misleading docs about timestamp_field location (#4873)

docs/configuration/index-config.md

@@ -680,7 +680,7 @@ explicitly in the schema, or may refer to a field captured by the dynamic mode.
 
 ## Retention policy
 
-This section describes how Quickwit manages data retention. In Quickwit, the retention policy manager drops data on a split basis as opposed to individually dropping documents. Splits are evaluated based on their `time_range` which is derived from the index timestamp field specified in the (`indexing_settings.timestamp_field`) settings. Using this setting, the retention policy will delete a split when `now() - split.time_range.end >= retention_policy.period`
+This section describes how Quickwit manages data retention. In Quickwit, the retention policy manager drops data on a split basis as opposed to individually dropping documents. Splits are evaluated based on their `time_range` which is derived from the index timestamp field specified in the (`doc_mapping.timestamp_field`) settings. Using this setting, the retention policy will delete a split when `now() - split.time_range.end >= retention_policy.period`
 
 ```yaml
 version: 0.7

quickwit/quickwit-config/src/index_config/mod.rs

@@ -561,7 +561,7 @@ pub(super) fn validate_index_config(
 
         ensure!(
             doc_mapping.timestamp_field.is_some(),
-            "retention policy requires a timestamp field, but indexing settings do not declare one"
+            "retention policy requires a timestamp field, but doc mapping does not declare one"
         );
     }
     Ok(())