systemd: add explanation to -Dmode

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@12066 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

chapter08/systemd.xml

@@ -190,6 +190,14 @@ meson --prefix=/usr                 \
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><parameter>-Dmode=release</parameter></term>
+        <listitem>
+          <para>Disable some features considered experimental by upstream.
+          </para>
+        </listitem>
+      </varlistentry>
+
     </variablelist>
 
     <para>Compile the package:</para>