Fixes #4272 Documentation for the new PropertyNormalizer

Fixes #4272: Documentation for the new PropertyNormalizer introduced in symfony/symfony#9708

This is my first time with RST I believe, be nice :smile: 
I prefer to use the definition list syntax here:

````rst
The :class:`Symfony\\Component\\Serializer\\Normalizer\\GetSetMethodNormalizer`
    This normalizer [...]

The :class:`Symfony\\Component\\Serializer\\Normalizer\\PropertyNormalizer`
    [...]
````