Update max_concurrent_shard_request parameter docs (elastic#42227)

Some of the docs were outdated as they did not mention that the limit is
not per node. Also, The default value changed.

Relates to elastic#31206

docs/reference/search.asciidoc

@@ -154,12 +154,11 @@ configure a soft limit, you can update the `action.search.shard_count.limit`
 cluster setting in order to reject search requests that hit too many shards.
 
 The request parameter `max_concurrent_shard_requests` can be used to control the
-maximum number of concurrent shard requests the search API will execute for the
-request. This parameter should be used to protect a single request from
+maximum number of concurrent shard requests the search API will execute per node
+for the request. This parameter should be used to protect a single request from
 overloading a cluster (e.g., a default request will hit all indices in a cluster
 which could cause shard request rejections if the number of shards per node is
-high). This default is based on the number of data nodes in the cluster but at
-most `256`.
+high). This default value is `5`.
 
 --
 

docs/reference/search/multi-search.asciidoc

@@ -85,15 +85,16 @@ The msearch's `max_concurrent_searches` request parameter can be used to control
 the maximum number of concurrent searches the multi search api will execute.
 This default is based on the number of data nodes and the default search thread pool size.
 
-The request parameter `max_concurrent_shard_requests` can be used to control the
-maximum number of concurrent shard requests the each sub search request will execute.
-This parameter should be used to protect a single request from overloading a cluster
-(e.g., a default request will hit all indices in a cluster which could cause shard request rejections
-if the number of shards per node is high). This default is based on the number of
-data nodes in the cluster but at most `256`.In certain scenarios parallelism isn't achieved through
-concurrent request such that this protection will result in poor performance. For
-instance in an environment where only a very low number of concurrent search requests are expected
-it might help to increase this value to a higher number.
+The request parameter `max_concurrent_shard_requests` can be used to control
+the maximum number of concurrent shard requests that each sub search request
+will execute per node. This parameter should be used to protect a single
+request from overloading a cluster (e.g., a default request will hit all
+indices in a cluster which could cause shard request rejections if the number
+of shards per node is high). This default value is `5`.In certain scenarios
+parallelism isn't achieved through concurrent request such that this protection
+will result in poor performance. For instance in an environment where only a
+very low number of concurrent search requests are expected it might help to
+increase this value to a higher number.
 
 [float]
 [[msearch-security]]

rest-api-spec/src/main/resources/rest-api-spec/api/msearch.json

@@ -32,8 +32,8 @@
         },
         "max_concurrent_shard_requests" : {
           "type" : "number",
-          "description" : "The number of concurrent shard requests each sub search executes concurrently. This value should be used to limit the impact of the search on the cluster in order to limit the number of concurrent shard requests",
-          "default" : "The default grows with the number of nodes in the cluster but is at most 256."
+          "description" : "The number of concurrent shard requests each sub search executes concurrently per node. This value should be used to limit the impact of the search on the cluster in order to limit the number of concurrent shard requests",
+          "default" : 5
         },
         "rest_total_hits_as_int" : {
           "type" : "boolean",

rest-api-spec/src/main/resources/rest-api-spec/api/search.json

@@ -181,7 +181,7 @@
         "max_concurrent_shard_requests" : {
           "type" : "number",
           "description" : "The number of concurrent shard requests per node this search executes concurrently. This value should be used to limit the impact of the search on the cluster in order to limit the number of concurrent shard requests",
-          "default" : "The default is 5."
+          "default" : 5
         },
         "pre_filter_shard_size" : {
           "type" : "number",