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: 
you should indent this paragraph (and the next one) with the same level as the text on line 246. This way, it'll all be put under the first list item. Currently, you have 2 lists which contains just one item.