Add missing HOUR TO SECOND option to list of possible INTERVAL field sets,

as noted by Sebastien Flaesch.  Also update the claim that we simply throw
away fields outside this set --- that got changed later to only discard
less-significant fields.

doc/src/sgml/datatype.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/datatype.sgml,v 1.239 2009/06/17 21:58:49 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/datatype.sgml,v 1.240 2009/07/08 17:21:55 tgl Exp $ -->
 
  <chapter id="datatype">
   <title id="datatype-title">Data Types</title>
@@ -1524,11 +1524,11 @@ SELECT b, char_length(b) FROM test2;
     DAY TO MINUTE
     DAY TO SECOND
     HOUR TO MINUTE
+    HOUR TO SECOND
     MINUTE TO SECOND
 </programlisting>
-    Input falling outside the specified set of fields is silently discarded.
     Note that if both <replaceable>fields</replaceable> and
-    <replaceable>precision</replaceable> are specified, the
+    <replaceable>p</replaceable> are specified, the
     <replaceable>fields</replaceable> must include <literal>SECOND</>,
     since the precision applies only to the seconds.
    </para>
@@ -2468,11 +2468,15 @@ P <optional> <replaceable>years</>-<replaceable>months</>-<replaceable>days</> <
 
     <para>
      When writing an interval constant with a <replaceable>fields</>
-     specification, or when assigning to an interval column that was defined
-     with a <replaceable>fields</> specification, the interpretation of
+     specification, or when assigning a string to an interval column that was
+     defined with a <replaceable>fields</> specification, the interpretation of
      unmarked quantities depends on the <replaceable>fields</>.  For
      example <literal>INTERVAL '1' YEAR</> is read as 1 year, whereas
-     <literal>INTERVAL '1'</> means 1 second.
+     <literal>INTERVAL '1'</> means 1 second.  Also, field values
+     <quote>to the right</> of the least significant field allowed by the
+     <replaceable>fields</> specification are silently discarded.  For
+     example, writing <literal>INTERVAL '1 day 2:03:04' HOUR TO MINUTE</>
+     results in dropping the seconds field, but not the day field.
     </para>
 
     <para>