man: mention that /var/lib/systemd/timesync/clock might have the compilation timestamp

https://bugs.freedesktop.org/show_bug.cgi?id=90085

man/systemd-timesyncd.service.xml

@@ -85,7 +85,8 @@
 
         <listitem>
           <para>The modification time of this file indicates the timestamp of the last successful
-          synchronization.</para>
+          synchronization (or at least the systemd build date, in case synchronization was not
+          possible).</para>
         </listitem>
       </varlistentry>