Add REST API spec for /_search_shards endpoint

Also adds REST test for /_search_shards

Closes elastic#5907

rest-api-spec/api/search_shards.json

@@ -0,0 +1,51 @@
+{
+  "search_shards": {
+    "documentation": "http://www.elasticsearch.org/guide/en/elasticsearch/reference/master/search-shards.html",
+    "methods": ["GET", "POST"],
+    "url": {
+      "path": "/{index}/{type}/_search_shards",
+      "paths": ["/_search_shards", "/{index}/_search_shards", "/{index}/{type}/_search_shards"],
+      "parts": {
+        "index": {
+          "type" : "string",
+          "required" : false,
+          "description" : "The name of the index"
+        },
+        "type": {
+          "type" : "string",
+          "required" : false,
+          "description" : "The type of the document"
+        }
+      },
+      "params": {
+        "preference": {
+          "type" : "string",
+          "description" : "Specify the node or shard the operation should be performed on (default: random)"
+        },
+        "routing": {
+          "type" : "string",
+          "description" : "Specific routing value"
+        },
+        "local": {
+          "type": "boolean",
+          "description": "Return local information, do not retrieve the state from master node (default: false)"
+        },
+        "ignore_unavailable": {
+          "type" : "boolean",
+          "description" : "Whether specified concrete indices should be ignored when unavailable (missing or closed)"
+        },
+        "allow_no_indices": {
+           "type" : "boolean",
+           "description" : "Whether to ignore if a wildcard indices expression resolves into no concrete indices. (This includes `_all` string or when no indices have been specified)"
+        },
+        "expand_wildcards": {
+            "type" : "enum",
+            "options" : ["open","closed"],
+            "default" : "open",
+            "description" : "Whether to expand wildcard expression to concrete indices that are open, closed or both."
+        }
+      }
+    },
+    "body": null
+  }
+}

rest-api-spec/test/search_shards/10_basic.yaml

@@ -0,0 +1,16 @@
+---
+"Basic /_search_shards test":
+  - do:
+      indices.create:
+        index: test_1
+
+  - do:
+      cluster.health:
+        wait_for_status: yellow
+
+  - do:
+      search_shards:
+        index:  test_1
+        routing: foo
+
+  - match: { shards.0.0.index: test_1 }