Document the addition of debug:vmod_so_keep to 5.2.

doc/sphinx/whats-new/upgrading-5.2.rst

@@ -21,6 +21,9 @@ We have updated the documentation for :ref:`ref_param_send_timeout`,
 :ref:`ref_param_idle_send_timeout`, :ref:`ref_param_timeout_idle` and
 :ref:`ref_param_ban_cutoff`.
 
+Added the debug bit ``vmod_so_keep``, see :ref:`ref_param_debug` and
+the notes about changes for developers below.
+
 Changes to VCL
 ==============
 
@@ -137,6 +140,11 @@ Other changes
     development of client apps, is now publicly available, see
     :ref:`whatsnew_vut_5.2`.
 
+  * The debug bit ``vmod_so_keep`` instructs Varnish not to clean
+    up its copies of VMOD shared objects when it stops. This makes
+    it possible for VMOD authors to load their code into a debugger
+    after a varnishd crash. See :ref:`ref_param_debug`.
+
   * *XXX: anything else, such as sanitizer flags?*
 
   * *XXX: ...*