config: expand description of PHP_EXECUTABLE

config.php-dist

@@ -50,7 +50,10 @@
 	// *****************************
 
 	define('PHP_EXECUTABLE', '/usr/bin/php');
-	// Path to PHP executable, used for various command-line tt-rss programs
+	// Path to PHP *COMMAND LINE* executable, used for various command-line tt-rss programs and
+	// update daemon. Do not try to use CGI binary here, it won't work. If you see HTTP headers
+	// being displayed while running tt-rss scripts, then most probably you are using the CGI
+	// binary. If you are unsure what to put in here, ask your hosting provider.
 
 	define('LOCK_DIRECTORY', 'lock');
 	// Directory for lockfiles, must be writable to the user you run