DOC: Add code example including ID HiddenField to SingleRecordAdmin

[lozcalver]

Description

Issues

• SingleRecordAdmin doesn't render the edit form properly silverstripe-admin#2057

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[GuySartorelli]

Adding this to the top of SingleRecordAdmin::getEditForm() will mean people don't have to add this extra field.

if (!$fields) {
    if (!$id) {
        $id = $this->currentRecordID();
    }
    $record = $this->getRecord($id);
    if ($record) {
        $fields = $record->getCMSFields();
    }
}
if ($fields && !$fields->dataFieldByName('ID')) {
    $fields->add(HiddenField::create('ID'));
}

Given a record could be edited in different contexts, we shouldn't require adding a field to the model itself just so it can be edited correctly in this one specific context. We should just make SingleRecordAdmin work out of the box instead.

From what I can tell, the above snippet works for new admins and doesn't break existing ones since it checks for the existence of the ID field initially.

[lozcalver]

Good idea!