Update multi-get.asciidoc

Documented that the fields parameter can be passed in the query string.

Closes #4006

docs/reference/docs/multi-get.asciidoc

@@ -178,6 +178,26 @@ curl 'localhost:9200/_mget' -d '{
 }'
 --------------------------------------------------
 
+Alternatively, you can specify the `fields` parameter in the query string
+as a default to be applied to all documents.
+
+[source,js]
+--------------------------------------------------
+curl 'localhost:9200/test/type/_mget?fields=field1,field2' -d '{
+    "docs" : [
+        {
+            "_id" : "1" <1>
+        },
+        {
+            "_id" : "2",
+            "fields" : ["field3", "field4"] <2>
+        }
+    ]
+}'
+--------------------------------------------------
+<1> Returns `field1` and `field2`
+<2> Returns `field3` and `field4`
+
 [float]
 === Generated fields