add buttons_callback for selectAll and editAll views to callback documentation

[bezin]

The extension generator includes two templates for callback that are not documented yet:

    // Select
    'select' => array
    (
        'buttons_callback' => array()
    ),

    // Edit
    'edit' => array
    (
        'buttons_callback' => array()
    ),

With these callbacks its possible to add buttons after selecting and editing multiple datasets, eg. the "Generate Alias" button in tl_page.