document rootfs options

doc/lxc.conf.sgml.in

@@ -373,8 +373,8 @@ Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
     <refsect2>
       <title>Root file system</title>
       <para>
-	The root file system is the location where the container will
-	chroot.
+	The root file system of the container can be different than that
+	of the host system.
       </para>
       <variablelist>
 	<varlistentry>
@@ -383,8 +383,42 @@ Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 	  </term>
 	  <listitem>
 	    <para>
-	      specify a file location containing the new file tree for
-	      a root file system.
+	      specify a directory to become the root of the container.
+	      If not specified, the container shares its root file
+	      system with the host.
+	    </para>
+	  </listitem>
+	</varlistentry>
+
+	<varlistentry>
+	  <term>
+	    <option>lxc.rootfs.mount</option>
+	  </term>
+	  <listitem>
+	    <para>
+	      where to recursively bind <option>lxc.rootfs</option>
+	      before pivoting.  This is to ensure success of the
+	      <citerefentry>
+		<refentrytitle><command>pivot_root</command></refentrytitle>
+		<manvolnum>8</manvolnum>
+	      </citerefentry>
+	      syscall.  Any directory suffices, the default should
+	      generally work.
+	    </para>
+	  </listitem>
+	</varlistentry>
+
+	<varlistentry>
+	  <term>
+	    <option>lxc.pivotdir</option>
+	  </term>
+	  <listitem>
+	    <para>
+	      where to pivot the original root file system under
+	      <option>lxc.rootfs</option>, specified relatively to
+	      that.  The default is <filename>oldrootfs</filename>.
+	      It is created if necessary, and also removed after
+	      unmounting everything from it during container setup.
 	    </para>
 	  </listitem>
 	</varlistentry>