Document proper usage of toMany collections (denormalization + validation)

[soyuka]

See api-platform/core#1534

@bendavies what's the validation trick? Put the validation on the property instead of the getter?

[bendavies]

Accessor must call getValues:

public function getFoo(): array
{
    return $this->foos->getValues();
}

validation:
replace

<property name="foos">
    <constraint name="Valid"/>
</property>

with

<getter property="foos">
    <constraint name="Valid"/>
</getter>

or the equivalent if you are using yaml/annotations.

[soyuka]

#740 finally took time to document that :D