Correct documentation about embedded forms

[webmozart]

Currently, the documentation about embedded forms is incorrect. The documentation states that the "cascade_validation" option should be used to enable validation of the embedded form. This is wrong. Instead, one should set the Valid constraint on the respective property in the model.

The "cascade_validation" option was introduced to solve a very rare use case, namely when embedded forms have the "mapped" option set to false. This use case can also be solved using the Valid constraint as of 2.5. Thus "cascade_validation" will be deprecated in 2.6.

More information in the deprecation ticket: symfony/symfony#12237

[peterrehm]

Just submitted the PR.

[webmozart]

Wow, that was quick! Thanks @peterrehm :)

[peterrehm]

No problem. That was an easy pick :)

[wouterj]

Fixed already by #4348