Document that we deviate from ISO 8601 by not using 'T' on output.

Per discussion, we should explain that we follow RFC 3339 and not really
the letter of the ISO 8601 spec for timestamp output format.  Mostly
Brendan Jurd's wording, though I tweaked it to clarify that we do take 'T'
on input.  Minor additional copy-editing and markup-tweaking, too.

doc/src/sgml/datatype.sgml

@@ -2206,37 +2206,46 @@ January 8 04:05:06 1999 PST
        </thead>
        <tbody>
         <row>
-         <entry>ISO</entry>
-         <entry>ISO 8601/SQL standard</entry>
-         <entry>1997-12-17 07:37:16-08</entry>
+         <entry><literal>ISO</literal></entry>
+         <entry>ISO 8601, SQL standard</entry>
+         <entry><literal>1997-12-17 07:37:16-08</literal></entry>
         </row>
         <row>
-         <entry>SQL</entry>
+         <entry><literal>SQL</literal></entry>
          <entry>traditional style</entry>
-         <entry>12/17/1997 07:37:16.00 PST</entry>
+         <entry><literal>12/17/1997 07:37:16.00 PST</literal></entry>
         </row>
         <row>
-         <entry>POSTGRES</entry>
+         <entry><literal>Postgres</literal></entry>
          <entry>original style</entry>
-         <entry>Wed Dec 17 07:37:16 1997 PST</entry>
+         <entry><literal>Wed Dec 17 07:37:16 1997 PST</literal></entry>
         </row>
         <row>
-         <entry>German</entry>
+         <entry><literal>German</literal></entry>
          <entry>regional style</entry>
-         <entry>17.12.1997 07:37:16.00 PST</entry>
+         <entry><literal>17.12.1997 07:37:16.00 PST</literal></entry>
         </row>
        </tbody>
       </tgroup>
      </table>
 
+    <note>
+     <para>
+      ISO 8601 specifies the use of uppercase letter <literal>T</> to separate
+      the date and time.  <productname>PostgreSQL</> accepts that format on
+      input, but on output it uses a space rather than <literal>T</>, as shown
+      above.  This is for readability and for consistency with RFC 3339 as
+      well as some other database systems.
+     </para>
+    </note>
+
     <para>
      In the <acronym>SQL</acronym> and POSTGRES styles, day appears before
      month if DMY field ordering has been specified, otherwise month appears
      before day.
      (See <xref linkend="datatype-datetime-input">
      for how this setting also affects interpretation of input values.)
-     <xref linkend="datatype-datetime-output2-table"> shows an
-     example.
+     <xref linkend="datatype-datetime-output2-table"> shows examples.
     </para>
 
      <table id="datatype-datetime-output2-table">
@@ -2253,29 +2262,33 @@ January 8 04:05:06 1999 PST
         <row>
          <entry><literal>SQL, DMY</></entry>
          <entry><replaceable>day</replaceable>/<replaceable>month</replaceable>/<replaceable>year</replaceable></entry>
-         <entry>17/12/1997 15:37:16.00 CET</entry>
+         <entry><literal>17/12/1997 15:37:16.00 CET</literal></entry>
         </row>
         <row>
          <entry><literal>SQL, MDY</></entry>
          <entry><replaceable>month</replaceable>/<replaceable>day</replaceable>/<replaceable>year</replaceable></entry>
-         <entry>12/17/1997 07:37:16.00 PST</entry>
+         <entry><literal>12/17/1997 07:37:16.00 PST</literal></entry>
         </row>
         <row>
          <entry><literal>Postgres, DMY</></entry>
          <entry><replaceable>day</replaceable>/<replaceable>month</replaceable>/<replaceable>year</replaceable></entry>
-         <entry>Wed 17 Dec 07:37:16 1997 PST</entry>
+         <entry><literal>Wed 17 Dec 07:37:16 1997 PST</literal></entry>
         </row>
        </tbody>
       </tgroup>
      </table>
 
     <para>
-     The date/time styles can be selected by the user using the
+     The date/time style can be selected by the user using the
      <command>SET datestyle</command> command, the <xref
      linkend="guc-datestyle"> parameter in the
      <filename>postgresql.conf</filename> configuration file, or the
      <envar>PGDATESTYLE</envar> environment variable on the server or
-     client.  The formatting function <function>to_char</function>
+     client.
+    </para>
+
+    <para>
+     The formatting function <function>to_char</function>
      (see <xref linkend="functions-formatting">) is also available as
      a more flexible way to format date/time output.
     </para>