add explanation

version.php

@@ -1,6 +1,6 @@
 <?php
 
-// We only can count up. Reset minor/patchlevel when updating major/minor version number.
+// We only can count up. The 4. digit is only for the internal patchlevel to trigger DB upgrades between betas, final and RCs. This is _not_ the public version number. Reset minor/patchlevel when updating major/minor version number.
 $OC_Version=array(5, 80, 8, 0);
 
 // The human radable string