WordPress at a glance
filter is not described

wp_link_pages filter-hook . WP 3.6.0

Filters the HTML output of page links for paginated posts.

Usage

add_filter( 'wp_link_pages', 'filter_function_name_3322', 10, 2 );
function filter_function_name_3322( $output, $args ){
	// filter...

	return $output;
}
$output(string)
HTML output of paginated posts' page links.
$args(array)
An array of arguments.

Where the hook is called

wp_link_pages()
wp-includes/post-template.php 946
$html = apply_filters( 'wp_link_pages', $output, $args );

Where the hook is used (in WP core)

Does not used.