title: How to explain Composer's error log?
body: <p>I did a lot of research on the web, but did not find a documentation of the composer error log. In the discussions I found, nobody had an explanation that was consistent with the error log. For example:</p>
  
  <ul>
  <li><a href="https://github.com/composer/composer/issues/2702" rel="noreferrer">[Support] Need explanation for "Conclusion: don't install ..."</a></li>
  <li><a href="https://stackoverflow.com/q/45009772">Why composer says &quot;Conclusion: don&#39;t install&quot; when (seemingly) no obstacles are present?</a></li>
  </ul>
  
  <p>I know, what composer does and can resolve issues on my own, but I often have to consult packagist.org for this. Despite being quite (and unnecessarily) verbose, the composer log only gives me some hints. It does not really point out the concrete problems.</p>
  
  <p>Does anyone know of a complete documentation or how to explain the reasoning behind the logs, maybe taking the above ones as an example?</p>