man: fix wrong KillUserProcesses= default in systemd-run(1)

systemd-run(1) hard-coded "the default" wording for KillUserProcesses=, but the
actual compile-time default is determined by the -Ddefault-kill-user-processes=
meson build option, which distributions set differently at packaging time.

Consistent with logind.conf(5).

Fixes: #42805

Signed-off-by: Shihao Ren <renshihao.rsh@gmail.com>

man/systemd-run.xml

@@ -1,6 +1,9 @@
 <?xml version='1.0'?>
 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
-  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
+  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
+<!ENTITY % entities SYSTEM "custom-entities.ent" >
+%entities;
+]>
 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
 
 <refentry id="systemd-run"
@@ -708,9 +711,9 @@ There is a screen on:
       and a service unit would be terminated. Running <command>screen</command>
       as a user unit has the advantage that it is not part of the session scope.
       If <varname>KillUserProcesses=yes</varname> is configured in
-      <citerefentry><refentrytitle>logind.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
-      the default, the session scope will be terminated when the user logs
-      out of that session.</para>
+      <citerefentry><refentrytitle>logind.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+      (defaults to <literal>&KILL_USER_PROCESSES;</literal>), the session scope will be terminated when
+      the user logs out of that session.</para>
 
       <para>The <filename>user@.service</filename> is started automatically
       when the user first logs in, and stays around as long as at least one