Updated information about handling validation of embedded forms to Valid...

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4346

Applied Valid constraint instead of the cascade_validation option since this option is supposed to be deprecated with symfony/symfony#12237
Let's add a link to the section in the book chapter that you just added so people can see an example. What do you think?