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
ah yes, you can use `:ref:` to point to a specific "spot" (and you add a little `_foo-bar-baz` in the target spot). We have a few examples lying around in the docs - let me know if I'm being too vague :)