*** a/doc/src/sgml/ref/set_transaction.sgml
--- b/doc/src/sgml/ref/set_transaction.sgml
***************
*** 135,140 **** SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transa
--- 135,151 ----
     among those listed.  This is a high-level notion of read-only that
     does not prevent all writes to disk.
    </para>
+ 
+   <para>
+    The <literal>DEFERRABLE</literal> transaction property has no effect
+    unless the transaction is also <literal>SERIALIZABLE</literal> and
+    <literal>READ ONLY</literal>.  When all of these properties are set on a
+    transaction, the transaction may block when first acquiring its snapshot,
+    after which it is able to run without the normal overhead of a
+    <literal>SERIALIZABLE</literal> transaction and without any risk of
+    contributing to or being cancelled by a serialization failure.  This mode
+    is well suited for long-running reports or backups.
+   </para>
   </refsect1>
  
   <refsect1>