man: move os-release, machine-info, vconsole.conf vars to envvar section

Strictly speaking, those are not environment variables, but they are compatible
and people think about them like this. Moving them makes them easier to find.

man/machine-info.xml

@@ -58,7 +58,7 @@
     <para>The following machine metadata parameters may be set using
     <filename>/etc/machine-info</filename>:</para>
 
-    <variablelist>
+    <variablelist class='environment-variables'>
 
       <varlistentry>
         <term><varname>PRETTY_HOSTNAME=</varname></term>

man/os-release.xml

@@ -90,7 +90,7 @@
     <para>The following OS identifications parameters may be set using
     <filename>os-release</filename>:</para>
 
-    <variablelist>
+    <variablelist class='environment-variables'>
 
       <varlistentry>
         <term><varname>NAME=</varname></term>

man/vconsole.conf.xml

@@ -68,7 +68,7 @@
 
     <para>The following options are understood:</para>
 
-    <variablelist>
+    <variablelist class='environment-variables'>
 
       <varlistentry>
         <term><varname>KEYMAP=</varname></term>

tools/make-directive-index.py

@@ -46,8 +46,8 @@ TEMPLATE = '''\
         <refsect1>
                 <title>Environment variables</title>
 
-                <para>Environment variables understood by the systemd
-                manager and other programs.</para>
+                <para>Environment variables understood by the systemd manager
+                and other programs and environment variable-compatible settings.</para>
 
                 <variablelist id='environment-variables' />
         </refsect1>