Added docs for batched_reduce_size

Relates to #23288

Author: clintongormley

docs/reference/search/request-body.asciidoc

@@ -93,6 +93,14 @@ And here is a sample response:
     the query execution has actually terminated_early. Defaults to no
     terminate_after.
 
+`batched_reduce_size`::
+
+    The number of shard results that should be reduced  at once on the
+    coordinating node. This value should be used as a protection  mechanism to
+    reduce the memory overhead per search request if the potential  number of
+    shards in the request can be large.
+
+
 
 Out of the above, the `search_type` and the `request_cache` must be passed as
 query-string parameters. The rest of the search request should be passed

docs/reference/search/uri-request.asciidoc

@@ -67,6 +67,11 @@ query.
 |`analyze_wildcard` |Should wildcard and prefix queries be analyzed or
 not. Defaults to `false`.
 
+|`batched_reduce_size` | The number of shard results that should be reduced
+at once on the coordinating node. This value should be used as a protection
+mechanism to reduce the memory overhead per search request if the potential
+number of shards in the request can be large.
+
 |`default_operator` |The default operator to be used, can be `AND` or
 `OR`. Defaults to `OR`.