Add `send_ajax_response()` to Output Class documentation

[RK311y]

Suggested Change

Add documentation for the send_ajax_response() method in the Output Class documentation.

Reasoning

Add details about this available method to developers.

Example:

$output = array(
    'this' => 'that',
    'foo' => 'bar'
);
ee()->output->send_ajax_response($output);