whats-new: Mention systemd changes

Refs varnishcache/pkg-varnish-cache#154

doc/sphinx/whats-new/changes-trunk.rst

@@ -205,9 +205,21 @@ log processing utilities.
 Platform Support
 ================
 
+CentOS
+~~~~~~
+
 With the End of Life of CentOS 8, there will be no more packages for Red Hat
 Enterprise Linux 8 and its derivatives. We track CentOS Stream in Continuous
 Integration so Varnish Cache is expected to work on this platform, but there
 will be no el8 packages from now on.
 
+systemd
+~~~~~~~
+
+The kill mode of the varnish service was changed from ``process`` to ``mixed``
+to ensure that the cache process is killed if the manager process is timed out
+by systemd. Otherwise, a race exists if the cache process where a restart is
+carried on before the old cache process exits, creating conflict on resources
+such as listen ports.
+
 *eof*

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

@@ -143,4 +143,15 @@ In the VMOD descriptor, it is possible to add the following stanza::
 
 This is a good occasion to revisit unfortunate name choices in existing VMODs.
 
+Platform Support
+================
+
+systemd
+~~~~~~~
+
+To make the selection of the main process deterministic for the kill mode, a
+PID file is now expected by default in the varnish service. In a setup where
+the service command for ``ExecStart`` is overridden, a ``-P`` option matching
+the ``PIDFile`` setting is needed.
+
 *eof*