[master] 34dbf94ca whats-new: A quick word on vdef.h changes

[Dridi Boukelmoune]

commit 34dbf94ca45be7f134758b101b8a7865ad94097e
Author: Dridi Boukelmoune <dridi.boukelmoune at gmail.com>
Date:   Fri Sep 3 18:05:25 2021 +0200

    whats-new: A quick word on vdef.h changes
    
    There's technically nothing breaking there that require upgrade notes,
    but it doesn't hurt, since it contains items related to both VNUM and
    VRE breaking changes.

diff --git a/doc/sphinx/whats-new/upgrading-trunk.rst b/doc/sphinx/whats-new/upgrading-trunk.rst
index 23d9d85f3..d748425b6 100644
--- a/doc/sphinx/whats-new/upgrading-trunk.rst
+++ b/doc/sphinx/whats-new/upgrading-trunk.rst
@@ -166,6 +166,21 @@ Combined with the Address Sanitizer it can help VMOD authors find memory
 handling issues like buffer overflows that could otherwise be missed on a
 regular workspace.
 
+``vdef.h``
+----------
+
+The ``vdef.h`` header is no longer self-contained, it includes ``stddef.h``.
+
+Since it is the first header that should be included when working with Varnish
+bindings, some definitions were promoted to ``vdef.h``:
+
+- a fallback for the ``__has_feature()`` macro in its absence
+- VRT macros for Structured Fields number limits
+- ``struct txt`` and its companion macros (the macros require ``vas.h`` too)
+
+This header is implicitly included by ``vrt.h`` and ``cache.h`` and should not
+concern VMOD authors.
+
 Workspace API
 -------------