Added the API definition for getting a document

The `_mget` API was extracted to a separate file

rest-api-spec/api/get.json

@@ -4,31 +4,48 @@
     "methods": ["GET", "HEAD", "POST"],
     "url": {
       "path": "/{index}/{type}/{id}",
-      "paths": ["/{index}/{type}/{id}", "/{index}/{type}/{id}/_source", "/_mget", "/{index}/_mget", "/{index}/{type}/_mget"],
+      "paths": ["/{index}/{type}/{id}", "/{index}/{type}/{id}/_source"],
       "parts": {
         "id": {
+          "type" : "string",
+          "required" : true,
+          "description" : "The document ID"
         },
         "index": {
+          "type" : "string",
+          "required" : true,
+          "description" : "The name of the index"
         },
         "type": {
+          "type" : "string",
+          "required" : true,
+          "description" : "The type of the document (use `_all` to fetch the first document matching the ID across all types)"
         }
       },
       "params": {
         "fields": {
-        },
-        "id": {
+          "type": "list",
+          "description" : "A comma-separated list of fields to return in the response"
         },
         "parent": {
+          "type" : "string",
+          "description" : "The ID of the parent document"
         },
         "preference": {
+          "type" : "string",
+          "description" : "Specify the shards the operation should be performed on (default: random shard)"
         },
         "realtime": {
+          "type" : "boolean",
+          "description" : "Specify whether to perform the operation in realtime or search mode"
         },
         "refresh": {
+          "type" : "boolean",
+          "description" : "Refresh the shard containing the document before performing the operation"
         },
         "routing": {
-        },
-        "type": {
+          "type" : "string",
+          "description" : "Specific routing value"
         }
       }
     },