doc: disambiguation of getMapping sample response in JS SDK (#280)

Disambiguate `getMapping` sample response in JS 6 SDK documentation.

Author: etrousset

src/sdk-reference/js/6/collection/get-mapping/index.md

@@ -34,6 +34,31 @@ Additional query options
 
 Resolves to an `object` representing the collection mapping.
 
+Example :
+```js
+{
+  "<index>": {             // The provided <index>
+    "mappings": {
+      "<collection>": {    // The provided <collection>
+        // The actual mapping of the collection starts here:
+        "properties": {
+          "field1": {
+            "type": "integer"
+          },
+          "field2": {
+            "type": "keyword"
+          },
+          "field3": {
+              "type":   "date",
+              "format": "yyyy-MM-dd HH:mm:ss||yyyy-MM-dd||epoch_millis"
+          }
+        }
+      }
+    }
+  }
+}
+```
+
 ## Usage
 
 [snippet=get-mapping]

src/sdk-reference/js/6/collection/get-mapping/snippets/get-mapping.js

@@ -1,21 +1,38 @@
 try {
   const mapping = await kuzzle.collection.getMapping('nyc-open-data', 'yellow-taxi');
   console.log(mapping);
+
   /*
-    {
-      properties: {
-        license: { type: 'keyword' },
-        driver: {
-          properties: {
-            name: { type: 'keyword' },
-            curriculum: { type: 'text' }
+  {
+    "nyc-open-data": {
+      "mappings": {
+        "yellow-taxi": {
+          "dynamic": "false",
+          "_meta": {
+            "area": "Panipokhari"
+          },
+          "properties": {
+            "license": {
+              "type": "keyword"
+            },
+            "driver": {
+              "properties": {
+                "name": {
+                  "type": "keyword"
+                },
+                "curriculum": {
+                  "type": "text"
+                }
+              }
+            }
           }
         }
       }
     }
+  }
   */
 
   console.log('Success');
 } catch (error) {
   console.error(error.message);
-}
+}