docs: fixes to Content Negotiation / Serialization (api-platform#1357)

Author: jamesisaac

core/content-negotiation.md

@@ -2,7 +2,7 @@
 
 The API system has built-in [content negotiation](https://en.wikipedia.org/wiki/Content_negotiation) capabilities.
 
-By default, only the [JSON-LD](https://json-ld.org) format is enabled. However API Platform Core supports many more formats and can be extended.
+By default, only the [JSON-LD](https://json-ld.org) and JSON formats are enabled. However API Platform Core supports many more formats and can be extended.
 
 The framework natively supports JSON-LD (and Hydra), GraphQL, JSON:API, HAL, YAML, CSV, HTML (API docs), raw JSON and raw XML.
 Using the raw JSON or raw XML formats is discouraged, prefer using JSON-LD instead, which provides more feature and is as easy to use.
@@ -52,6 +52,7 @@ api_platform:
         yaml:     ['application/x-yaml']
         csv:      ['text/csv']
         html:     ['text/html']
+        myformat: ['application/vnd.myformat']
 ```
 
 To enable GraphQL support, [read the dedicated chapter](graphql.md).

core/serialization.md

@@ -694,7 +694,9 @@ services:
 Note: this normalizer will work only for JSON-LD format, if you want to process JSON data too, you have to decorate another service:
 
 ```yaml
-    App\Serializer\ApiNormalizer:
+    # Need a different name to avoid duplicate YAML key
+    'app.serializer.normalizer.item.json':
+        class: 'App\Serializer\ApiNormalizer'
         decorates: 'api_platform.serializer.normalizer.item'
 ```