Adjust phpdocs to clarify that streamPdfFromHtml never returns

This commit also includes phpdoc formating changes.

Strict types have been left out on purpose to be compatible with the
already existing implementation.

Author: TheSyscall

library/Icinga/Application/Hook/PdfexportHook.php

@@ -7,6 +7,7 @@
 
 use Icinga\Application\Logger;
 use Icinga\Exception\IcingaException;
+use ipl\Html\ValidHtml;
 use RuntimeException;
 use Throwable;
 
@@ -49,15 +50,17 @@ public static function first()
     /**
      * Get whether PDF export is supported
      *
-     * @return  bool
+     * @return bool
      */
     abstract public function isSupported();
 
     /**
      * Render the specified HTML to PDF and stream it to the client
      *
-     * @param   string  $html       The HTML to render to PDF
-     * @param   string  $filename   The filename for the generated PDF
+     * @param ValidHtml $html The HTML to render to PDF
+     * @param string $filename The filename for the generated PDF
+     *
+     * @return never
      */
     abstract public function streamPdfFromHtml($html, $filename);