documentation of preserve existing settings

Author: glefloch

docs/reference/indices/update-settings.asciidoc

@@ -4,8 +4,8 @@
 Change specific index level settings in real time.
 
 The REST endpoint is `/_settings` (to update all indices) or
-`{index}/_settings` to update one (or more) indices settings. The body
-of the request includes the updated settings, for example:
+`{index}/_settings` to update one (or more) indices settings.
+The body of the request includes the updated settings, for example:
 
 [source,js]
 --------------------------------------------------
@@ -21,6 +21,8 @@ PUT /twitter/_settings
 
 The list of per-index settings which can be updated dynamically on live
 indices can be found in <<index-modules>>.
+To preserve existing settings from being updated, the `preserve_existing`
+request parameter can be set to `true`.
 
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