man: document default rlimits

Fixes #19645

man/custom-entities.ent.in

@@ -11,5 +11,6 @@
 <!ENTITY KILL_USER_PROCESSES "{{ 'yes' if KILL_USER_PROCESSES else 'no' }}">
 <!ENTITY DEBUGTTY "{{DEBUGTTY}}">
 <!ENTITY RC_LOCAL_PATH "{{RC_LOCAL_PATH}}">
+<!ENTITY HIGH_RLIMIT_NOFILE "{{HIGH_RLIMIT_NOFILE}}">
 <!ENTITY fedora_latest_version "34">
 <!ENTITY fedora_cloud_release "1.2">

man/systemd-system.conf.xml

@@ -408,7 +408,23 @@
         <varname>LimitXXX=</varname> directives and they accept the same parameter syntax,
         see <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
         for details. Note that these resource limits are only defaults
-        for units, they are not applied to the service manager process (i.e. PID 1) itself.</para></listitem>
+        for units, they are not applied to the service manager process (i.e. PID 1) itself.</para>
+
+        <para>Most of these settings are unset, which means the resource limits are inherited from the kernel or, if
+        invoked in a container, from the container manager. However, the following have defaults:</para>
+        <itemizedlist>
+          <listitem><para><varname>DefaultLimitNOFILE=</varname> defaults to <literal>1024:&HIGH_RLIMIT_NOFILE;</literal>.
+          </para></listitem>
+
+          <listitem><para><varname>DefaultLimitCORE=</varname> does not have a default but it is worth mentioning that
+          <varname>RLIMIT_CORE</varname> is set to <literal>infinity</literal> by PID 1 which is inherited by its
+          children.</para></listitem>
+
+          <listitem><para>Note that the service manager internally increases <varname>RLIMIT_MEMLOCK</varname> for
+          itself, however the limit is reverted to the original value for child processes forked off.</para></listitem>
+        </itemizedlist>
+
+      </listitem>
       </varlistentry>
 
       <varlistentry>