Add docs about fields method and functions

[frozzare]

Source:

• wp-papi/papi@ea086db

Code example:

$fields = papi_get_fields($post_id);

[
	'name' => 'Fredrik'
]

// Same as
$name = papi_get_field('name');

// Modify fields.
class Simple_Page_Type extends Papi_Page_Type {
  public function fields($fields) {
    $fields['name'] = 'Test';
    return $fields;
  }
}

$fields = papi_get_fields($post_id);

[
	'name' => 'Test'
]