man: sd_journal_send does nothing when journald is not available

https://bugzilla.redhat.com/show_bug.cgi?id=1096067
(cherry picked from commit bdf9fc1a94)

man/sd_journal_print.xml

@@ -218,6 +218,11 @@ sd_journal_send("MESSAGE=Hello World, this is PID %lu!", (unsigned long) getpid(
                 errno-style error code. The
                 <citerefentry><refentrytitle>errno</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                 variable itself is not altered.</para>
+
+                <para>If
+                <citerefentry><refentrytitle>systemd-journald</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+                is not running (the socket is not present), those
+                functions do nothing, and also return 0.</para>
         </refsect1>
 
         <refsect1>