Improve documentation of single.php

<?php

/**

 * Template for single posts.

 *

 * WordPress identifies this file by its name (which MUST be single.php)

 * and displays it whenever a single post is displayed.

 * Template for a single post.

 *

 * Posts are generally used to present a piece of information that is relevant 

 * during a certain timespan, like a news article. After that time, the contents

 * during a certain timespan, like a news article or an announcement. After that time, the contents

 * often lose most of its relevance, aren't updated anymore (even if it means information

 * presented in a post stops to be accurate due to its old age) and kept only for

 * archival purposes.

 * Posts display an author and a post date.

 * 

 * Pages, on the other hand, contain information that is always relevant and should

 * always be kept up to date, see page.php for more.

 * Posts have an announcement-like character, and announcements are nothing else then messages,

 * and for messages, we generally care about the author, so we display it. 

 * 

 * WordPress identifies this file by its name (which MUST be single.php).

 *

 * @see https://codex.wordpress.org/Theme_Development#Single_Post_.28single.php.29

 */