[master] 2897c7e47 Mention VMOD immutability in "Upgrading for developers".

[Geoff Simmons]

commit 2897c7e477ee70184189bed56bf74f24ebc3bb2a
Author: Geoff Simmons <geoff at uplex.de>
Date:   Thu Mar 14 22:48:05 2019 +0100

    Mention VMOD immutability in "Upgrading for developers".

diff --git a/doc/sphinx/whats-new/upgrading-6.2.rst b/doc/sphinx/whats-new/upgrading-6.2.rst
index 9d816184c..011bd875a 100644
--- a/doc/sphinx/whats-new/upgrading-6.2.rst
+++ b/doc/sphinx/whats-new/upgrading-6.2.rst
@@ -187,4 +187,10 @@ defines macros for prepending the vmod prefix, and for naming enums
 and argument structs. For details, see the `change log`_, and examine
 the contents of ``vcc_if.h`` after generation.
 
+Going forward, we will adhere to the principle that data returned by
+VMOD methods and functions are immutable. This is now enforced in some
+places by use of the ``const`` modifier. A VMOD is free to do as it
+sees fit within its own implementation, but if you attempt to change
+something returned by another VMOD, the results are undefined.
+
 *eof*