man: describe the cgroup hierarchy kernel commandline options

Fixes #4827.

v2:
- update for the inverted interpration of systemd.legacy_systemd_cgroup_controller

man/kernel-command-line.xml

@@ -92,6 +92,8 @@
         <term><varname>systemd.default_standard_error=</varname></term>
         <term><varname>systemd.setenv=</varname></term>
         <term><varname>systemd.machine_id=</varname></term>
+        <term><varname>systemd.unified_cgroup_hierarchy</varname></term>
+        <term><varname>systemd.legacy_systemd_cgroup_controller</varname></term>
         <listitem>
           <para>Parameters understood by the system and service
           manager to control system behavior. For details, see

man/systemd.xml

@@ -1010,6 +1010,41 @@
         for every boot.</para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>systemd.unified_cgroup_hierarchy</varname></term>
+
+        <listitem><para>When specified without an argument or with a true argument,
+        enables the usage of
+        <ulink url="https://www.kernel.org/doc/Documentation/cgroup-v2.txt">unified cgroup hierarchy</ulink>
+        (a.k.a. cgroups-v2). When specified with a false argument, fall back to
+        hybrid or full legacy cgroup hierarchy.</para>
+
+        <para>If this option is not specified, the default behaviour is determined
+        during compilation (the <option>--with-default-hierarchy=</option>
+        option). If the kernel does not support unified cgroup hierarchy, the legacy
+        hierarchy will be used even if this option is specified.</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>systemd.legacy_systemd_cgroup_controller</varname></term>
+
+        <listitem><para>Takes effect if the full unified cgroup hierarchy is not used
+        (see previous option). When specified without an argument or with a true
+        argument, disables the use of "hybrid" cgroup hierarchy (i.e. a cgroups-v2
+        tree used for systemd, and
+        <ulink url="https://www.kernel.org/doc/Documentation/cgroup-v1/">legacy
+        cgroup hierarchy</ulink>, a.k.a. cgroups-v1, for other controllers), and
+        forces a full "legacy" mode. When specified with a false argument, enables
+        the use of "hybrid" hierarchy.</para>
+
+        <para>If this option is not specified, the default behaviour is determined
+        during compilation (the <option>--with-default-hierarchy=</option>
+        option). If the kernel does not support unified cgroup hierarchy, the legacy
+        hierarchy will be used  even if this option is specified.</para>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>quiet</varname></term>