Add time_series_dimensions param to flattened docs (#95374)

docs/reference/data-streams/tsds.asciidoc

@@ -102,6 +102,8 @@ parameter:
 * <<number,`long`>>
 * <<number,`unsigned_long`>>
 
+For a flattened field, use the `time_series_dimensions` parameter to configure an array of fields as dimensions. For details refer to <<flattened-params,`flattened`>>.
+
 [[dimension-limits]]
 .Dimension limits
 ****

docs/reference/mapping/types/flattened.asciidoc

@@ -308,6 +308,12 @@ The following mapping parameters are accepted:
     whitespace when building a query for this field. Accepts `true` or `false`
     (default).
 
+`time_series_dimensions`::
+    (Optional, array of strings) A list of fields inside the flattened object, where each field is a dimension
+    of the time series. Each field is specified using the relative path from the
+    root field and does not include the root field name.
+
+
 [[flattened-synthetic-source]]
 ==== Synthetic `_source`