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: 
No I have not seen any explanation :( I think it was because there was only 1 normalize before, so that's why all the examples use the same. The behavior of the "GetSet" normalizer was kind of considered "default".