WordPress at a glance
filter is not described

robots_txt filter-hook . WP 3.0.0

Filters the robots.txt output.

Usage

add_filter( 'robots_txt', 'filter_function_name_8797', 10, 2 );
function filter_function_name_8797( $output, $public ){
	// filter...

	return $output;
}
$output(string)
Robots.txt output.
$public(true/false)
Whether the site is considered "public".

Where the hook is called

do_robots()
wp-includes/functions.php 1343
echo apply_filters( 'robots_txt', $output, $public );

Where the hook is used (in WP core)

Does not used.