How can I write comments/hidden text in a post?

Yes, using WordPress shortcodes you can create a quick annotation shortcode.

add_shortcode( 'annotation', 'wpse47489_private_annotation' );

function wpse47489_private_annotation( $args, $content="" ) {
    if ( !current_user_can( 'administrator' ) ) return '';
    return "<div class="hidden-annotation">$content</div>";
}

This will return an empty string for anyone but administrators. You can always return an empty string '' though, these annotations will be visible when editing the post.

Use in the content like so:

Lorem ipsum!

[annotation]find better introductory paragraph[/annotation]

…dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.