Document unatteded-upgrades blacklisted packages and link to the trac

ticket.

tsa/howto/upgrades.mdwn

@@ -98,6 +98,14 @@ Services setup with the new systemd-based startup system documented in
 There's a feature request ([bug #843778](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=843778)) to implement support for
 those services directly in needrestart.
 
+### Blacklisted packages
+
+At the moment the only blacklisted packages for unattended_upgrades are:
+
+- openvswitch-switch
+- openvswitch-common
+
+See ([bug #34185]https://trac.torproject.org/projects/tor/ticket/34185)
 
 ### Kernel upgrades and reboots