The comment is being viewed. This hook can be used to add additional data to the comment before theming.
$comment: Passes in the comment the action is being performed on.
$view_mode: View mode, e.g. 'full', 'teaser'...
$langcode: The language code used for rendering.
function hook_comment_view($comment, $view_mode, $langcode) {
// how old is the comment
$comment->time_ago = time() - $comment->changed;
}