Mention that --force is required to override an already existing default.target

This solves the bug bnc#868439

---
 man/systemctl.xml |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Index: systemd-218/man/systemctl.xml
===================================================================
--- systemd-218.orig/man/systemctl.xml
+++ systemd-218/man/systemctl.xml
@@ -446,7 +446,7 @@ along with systemd; If not, see <http://
         <term><option>--force</option></term>
 
         <listitem>
-          <para>When used with <command>enable</command>, overwrite
+          <para>When used with <command>enable</command> or <command>set-default</command>, overwrite
           any existing conflicting symlinks.</para>
 
           <para>When used with <command>halt</command>,
@@ -1187,6 +1187,8 @@ kobject-uevent 1 systemd-udevd-kernel.so
           <listitem>
             <para>Set the default target to boot into. Command links
             <filename>default.target</filename> to the given unit.</para>
+            <para>If combined with <option>--force</option>, override
+            an already existing symlink for the default target.</para>
           </listitem>
         </varlistentry>