Fix definition of FPM listen.backlog configuration

Author: bukka

install/fpm/configuration.xml

@@ -260,8 +260,9 @@
       </term>
       <listitem>
        <para>
-        Set listen(2) backlog. A value of '-1' means unlimited. Default value:
-        -1.
+        Set listen(2) backlog. A value of <literal>-1</literal> means maximum on BSD systems.
+        Default value: <literal>-1</literal> (FreeBSD or OpenBSD) or <literal>511</literal>
+        (Linux and other platforms).
        </para>
       </listitem>
      </varlistentry>