libvirtd.default: Document TCP/TLS usage

Document TCP/TLS usage in the libvirtd.default
file to stop libvirtd conffiles from being misleading.

Closes: #1050464
LP: #1960937

debian/libvirt-daemon-system.libvirtd.default

@@ -7,8 +7,10 @@
 # connects.
 #LIBVIRTD_ARGS="--timeout 120"
 
-# If systemd socket activation is disabled, then the following
-# can be used to listen on TCP/TLS sockets
+# If systemd socket activation is in use (this is the default),
+# then TCP/TLS connectivity should be turned on by enabling the
+# libvirtd-{tcp,tls}.socket units. If that's not the case (e.g.
+# when using sysvinit), this line can be uncommented instead.
 #LIBVIRTD_ARGS="--listen"
 
 # Override the QEMU/SDL default audio driver probing when