Add docs for afterMarshal event.

[markstory]

Refs cakephp/cakephp#14383

[dereuromark]

I fixed up the example in 67181ea#diff-630ee5f14cae91a8c66d6943b6ce9bfdR660

[markstory]

Thanks!

[garas]

date('d') is string .

Use either idate('d') or compare to '20'

[dereuromark]

(int) cast would also suffice, I fix that up.

[saeideng]

LGTM
only I think better to mention that $data and $options are unchangeable , here or in doc block
and better to show example between 2 field that on validation process it is impossible

if ($entity->name, 1) == 'J' && $entity->xyz === '123')
         $entity->setError('name', '......');

//or 
if ($entity->x === 1)
     $entity->setError('y', '....message.....');

though current example and details are fine 👍

[dereuromark]

I am not sure if we should really keep the $data
The main interest here is the entity itself and its fields. And that you can issue error messages and alike.
The data I do not see much use for, and it makes this callback different than all others (having 1 more arg).