Added the API definition for `_msearch`

rest-api-spec/api/msearch.json

@@ -0,0 +1,30 @@
+{
+  "search": {
+    "documentation": "http://www.elasticsearch.org/guide/reference/api/multi-search/",
+    "methods": ["GET", "POST"],
+    "url": {
+      "path": "/_msearch",
+      "paths": ["/_msearch", "/{index}/_msearch", "/{index}/{type}/_msearch"],
+      "parts": {
+        "index": {
+         "type" : "list",
+         "description" : "A comma-separated list of index names to use as default"
+        },
+        "type": {
+          "type" : "list",
+          "description" : "A comma-separated list of document types to use as default"
+        }
+      },
+      "params": {
+        "search_type": {
+          "type" : "enum",
+          "options" : ["query_then_fetch", "query_and_fetch", "dfs_query_then_fetch", "dfs_query_and_fetch", "count", "scan"],
+          "description" : "Search operation type"
+        }
+      }
+    },
+    "body": {
+      "description": "The request definitions (metadata-search request pairs), separated by newlines"
+    }
+  }
+}