Add user upgrading note for no_separation change

I added this to UPGRADING.INTERNALS, but it should also be
mentioned in UPGRADING, as it affects user-visible behavior.

[ci skip]

UPGRADING

@@ -470,6 +470,11 @@ PHP 8.0 UPGRADE NOTES
         // With
         usort($array, fn($a, $b) => $a <=> $b);
 
+  . Any functions accepting callbacks that are not explicitly specified to
+    accept parameters by reference will now warn if a callback with reference
+    parameters is used. Examples include array_filter() and array_reduce().
+    This was already the case for most, but not all, functions previously.
+
 - Sysvmsg:
   . msg_get_queue() will now return an SysvMessageQueue object rather than a
     resource. Return value checks using is_resource() should be replaced with