[feature/class-prefix] Rename auth => phpbb_auth

PHPBB3-10609

phpBB/docs/auth_api.html

@@ -86,7 +86,7 @@
 	<p>To use any methods contained with the <code>auth</code> class it first needs to be instantiated. This is best achieved early in the execution of the script in the following manner:</p>
 
 	<div class="codebox"><pre>
-$auth = new auth();
+$auth = new phpbb_auth();
 	</pre></div>
 
 	<p>Once an instance of the class has been created you are free to call the various methods it contains. Please note that should you wish to use the <code>auth_admin</code> methods you will need to instantiate this separately but in the same way.</p>