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've added a small introduction in the Usage part.

Like @xabbuh said, what normalizers do is explained at the beginning of the article. What I've added is to make more explicit that there is more than 1 normalizer (because all the examples use the original normalizer).