man: update pam_systemd and systemd-logind man pages a bit

This builds on @utezduyar's systemd#4640, but extends on it.

Fixes: systemd#4550
Replaces: systemd#4640

Author: poettering

man/pam_systemd.xml

@@ -59,40 +59,33 @@
     <citerefentry><refentrytitle>systemd-logind.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
     and hence the systemd control group hierarchy.</para>
 
-    <para>On login, this module ensures the following:</para>
+    <para>On login, this module — in conjunction with <filename>systemd-logind.service</filename> — ensures the
+    following:</para>
 
     <orderedlist>
-      <listitem><para>If it does not exist yet, the user runtime
-      directory <filename>/run/user/$USER</filename> is created and
-      its ownership changed to the user that is logging
-      in.</para></listitem>
-
-      <listitem><para>The <varname>$XDG_SESSION_ID</varname>
-      environment variable is initialized. If auditing is available
-      and <command>pam_loginuid.so</command> was run before this
-      module (which is highly recommended), the variable is
-      initialized from the auditing session id
-      (<filename>/proc/self/sessionid</filename>). Otherwise, an
+      <listitem><para>If it does not exist yet, the user runtime directory <filename>/run/user/$UID</filename> is
+      either created or mounted as new <literal>tmpfs</literal> file system with quota applied, and its ownership
+      changed to the user that is logging in.</para></listitem>
+
+      <listitem><para>The <varname>$XDG_SESSION_ID</varname> environment variable is initialized. If auditing is
+      available and <command>pam_loginuid.so</command> was run before this module (which is highly recommended), the
+      variable is initialized from the auditing session id (<filename>/proc/self/sessionid</filename>). Otherwise, an
       independent session counter is used.</para></listitem>
 
-      <listitem><para>A new systemd scope unit is created for the
-      session. If this is the first concurrent session of the user, an
-      implicit slice below <filename>user.slice</filename> is
-      automatically created and the scope placed into it. An instance
-      of the system service <filename>[email protected]</filename>, which
-      runs the systemd user manager instance, is started.
-      </para></listitem>
+      <listitem><para>A new systemd scope unit is created for the session. If this is the first concurrent session of
+      the user, an implicit per-user slice unit below <filename>user.slice</filename> is automatically created and the
+      scope placed into it. An instance of the system service <filename>[email protected]</filename>, which runs the
+      systemd user manager instance, is started.  </para></listitem>
     </orderedlist>
 
     <para>On logout, this module ensures the following:</para>
 
     <orderedlist>
       <listitem><para>If enabled in
       <citerefentry><refentrytitle>logind.conf</refentrytitle>
-      <manvolnum>5</manvolnum></citerefentry>, all processes of the
-      session are terminated. If the last concurrent session of a user
-      ends, the user's systemd instance will be terminated too, and so
-      will the user's slice unit.</para></listitem>
+      <manvolnum>5</manvolnum></citerefentry> (<varname>KillUserProcesses=</varname>), all processes of the session are
+      terminated. If the last concurrent session of a user ends, the user's systemd instance will be terminated too,
+      and so will the user's slice unit.</para></listitem>
 
       <listitem><para>If the last concurrent session of a user ends,
       the <varname>$XDG_RUNTIME_DIR</varname> directory and all its

man/systemd-logind.service.xml

@@ -60,8 +60,14 @@
     manages user logins. It is responsible for:</para>
 
     <itemizedlist>
-      <listitem><para>Keeping track of users and sessions, their
-      processes and their idle state</para></listitem>
+      <listitem><para>Keeping track of users and sessions, their processes and their idle state. This is implemented by
+      allocating a systemd slice unit for each user below <filename>user.slice</filename>, and a scope unit below it
+      for each concurrent session of a user. Also, a per-user service manager is started as system service instance of
+      <filename>[email protected]</filename> for each user logged in.</para></listitem>
+
+      <listitem><para>Generating and managing session IDs. If auditing is available and an audit session ID is set for
+      a session already, the session ID is initialized from it. Otherwise, an independent session counter is
+      used.</para></listitem>
 
       <listitem><para>Providing PolicyKit-based access for users to
       operations such as system shutdown or sleep</para></listitem>