FreeBSD write cache control

I have applied the attached patch to document FreeBSD drive write cache
control.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +
diff --git a/doc/src/sgml/wal.sgml b/doc/src/sgml/wal.sgml
new file mode 100644
index 96d4916..4d11190
*** a/doc/src/sgml/wal.sgml
--- b/doc/src/sgml/wal.sgml
***************
*** 85,92 ****
      <listitem>
        <para>
          On <productname>FreeBSD</>, IDE drives can be queried using
!         <command>atacontrol</command>, and SCSI drives using
!         <command>sdparm</command>.
        </para>
      </listitem>

--- 85,93 ----
      <listitem>
        <para>
          On <productname>FreeBSD</>, IDE drives can be queried using
!         <command>atacontrol</command> and write caching turned off using
!         <literal>hw.ata.wc=0</> in <filename>/boot/loader.conf</>;
!         SCSI drives use <command>sdparm</command>.
        </para>
      </listitem>