Add documentation to deserialize on existing objects.

[luishdez]

Q	A
Bug report?	no
Feature request?	yes
BC Break report?	no
RFC?	yes

I've review all issues and I've seen a lot of people struggling with this. This is a very common feature used to build APIs…

I've found and example in the issues using a custom InitializedObjectConstructor and then:

    jms_serializer.object_constructor:
        alias: jms_serializer.initialized_object_constructor
        public: false

    jms_serializer.initialized_object_constructor:
         class:        App\Serializer\InitializedObjectConstructor
         arguments:    ["@jms_serializer.unserialize_object_constructor"]

Then:

        $context = new DeserializationContext();
        $context->attributes->set('target', $document);

And I guess that probably this is not the best way to do it and it's not very intuitive. I bet a lot of people will thank a proper documentation and/or an easy way to do this.

[goetas]

Fixed by #1308

[yosvield]

How is the best way to deserialize on existing objects, i do not find in the documentation