man: document that journalctl --user requires Storage=persistent

Fixes: #25061
(cherry picked from commit cad8fa4)

Related: RHEL-31070

man/journalctl.xml

@@ -111,7 +111,11 @@
         </para>
 
         <para>The <option>--user</option> option affects how <option>--unit=</option> arguments are
-        treated. See <option>--unit=</option>.</para></listitem>
+        treated. See <option>--unit=</option>.</para>
+
+        <para>Note that <option>--user</option> only works if persistent logging is enabled, via the
+        <varname>Storage=</varname> setting in
+        <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
       </varlistentry>
 
       <varlistentry>

man/journald.conf.xml

@@ -95,6 +95,9 @@
         <filename>/var/log/journal/</filename>, as the <filename>systemd-journald@.service</filename> service
         file by default carries <varname>LogsDirectory=</varname>. To turn that off, add a unit file drop-in
         file that sets <varname>LogsDirectory=</varname> to an empty string.</para>
+
+        <para>Note that per-user journal files are not supported unless persistent storage is enabled, thus
+        making <command>journalctl --user</command> unavailable.</para>
         </listitem>
       </varlistentry>