WordPress at a glance
filter is not described

wp_link_pages_link filter-hook . WP 3.6.0

Filters the HTML output of individual page number links.

Usage

add_filter( 'wp_link_pages_link', 'filter_function_name_2443', 10, 2 );
function filter_function_name_2443( $link, $i ){
	// filter...

	return $link;
}
$link(string)
The page number HTML output.
$i(int)
Page number for paginated posts' page links.

Where the hook is called

wp_link_pages()
wp-includes/post-template.php 908
$link = apply_filters( 'wp_link_pages_link', $link, $i );
wp-includes/post-template.php 922
$output .= apply_filters( 'wp_link_pages_link', $link, $prev );
wp-includes/post-template.php 932
$output .= apply_filters( 'wp_link_pages_link', $link, $next );

Where the hook is used (in WP core)

Does not used.