Add mention that UTC really means UT1.

Backpatch to 9.1.

doc/src/sgml/catalogs.sgml

@@ -8341,7 +8341,9 @@
    The view <structname>pg_timezone_names</structname> provides a list
    of time zone names that are recognized by <command>SET TIMEZONE</>,
    along with their associated abbreviations, UTC offsets,
-   and daylight-savings status.
+   and daylight-savings status.  (Technically,
+   <productname>PostgreSQL</productname> uses <acronym>UT1</> rather
+   than UTC because leap seconds are not handled.)
    Unlike the abbreviations shown in <link
    linkend="view-pg-timezone-abbrevs"><structname>pg_timezone_abbrevs</structname></link>, many of these names imply a set of daylight-savings transition
    date rules.  Therefore, the associated information changes across local DST

doc/src/sgml/func.sgml

@@ -6898,7 +6898,9 @@ SELECT EXTRACT(SECOND FROM TIME '17:12:28.5');
        <para>
         The time zone offset from UTC, measured in seconds.  Positive values
         correspond to time zones east of UTC, negative values to
-        zones west of UTC.
+        zones west of UTC.  (Technically,
+        <productname>PostgreSQL</productname> uses <acronym>UT1</> because
+        leap seconds are not handled.)
        </para>
       </listitem>
      </varlistentry>