man: extend on the usec/sec discrepancy

Let's document the discrepancy between the Sec and USec suffixing of
unit files and D-Bus properties at three places: in "systemctl show"
(where it already was briefly mentioned), in the D-Bus interface
description (at one place at least, i.e. the most prominent of
properties that encapsulate time values, there are many more) and in the
general man page explaining time values.

By documenting this at all three places I think we now do as much as we
can do about this highlighting the discrepancy of the naming and the
reasons behind it.

Fixes: #2047

man/systemctl.xml

@@ -446,7 +446,11 @@ Jan 12 10:46:45 example.com bluetoothd[8900]: gatt-time-server: Input/output err
             current main process identifier as <literal>MainPID</literal> (which is runtime state), and time settings
             are always exposed as properties ending in the <literal>…USec</literal> suffix even if a matching
             configuration options end in <literal>…Sec</literal>, because microseconds is the normalized time unit used
-            by the system and service manager.</para>
+            internally by the system and service manager.</para>
+
+            <para>For details about many of these properties, see the documentation of the D-Bus interface
+            backing these properties, see
+            <citerefentry><refentrytitle>org.freedesktop.systemd1</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
           </listitem>
         </varlistentry>
         <varlistentry>