Add title, post content, and category like add_post_meta and update_post_meta

By using wp_update_post(), found in wp-includes/post.php:

// Update post 42
$post = array();
$post['ID'] = 42;
$post['post_category'] = array( ... );
$post['post_content' ] = 'This is the updated content.';
$post['post_title' ] = 'This is the updated title.';

// Update the post into the database
wp_update_post( $post );