Document that BBU's do not allow partial page writes to be safely turned

off unless they guarantee that all writes to the BBU arrive in 8kB chunks.

Per discussion with Greg Smith

doc/src/sgml/wal.sgml

@@ -167,10 +167,12 @@
    <productname>PostgreSQL</> periodically writes full page images to
    permanent WAL storage <emphasis>before</> modifying the actual page on
    disk. By doing this, during crash recovery <productname>PostgreSQL</> can
-   restore partially-written pages from WAL.  If you have a battery-backed disk
-   controller or file-system software that prevents partial page writes
-   (e.g., ZFS), you can safely turn off this page imaging by turning off the
-   <xref linkend="guc-full-page-writes"> parameter.
+   restore partially-written pages from WAL.  If you have file-system software
+   that prevents partial page writes (e.g., ZFS),  you can turn off
+   this page imaging by turning off the <xref
+   linkend="guc-full-page-writes"> parameter. Battery-Backed Unit
+   (BBU) disk controllers do not prevent partial page writes unless
+   they guarantee that data is written to the BBU as full (8kB) pages.
   </para>
  </sect1>