doc: remove mention of UT1 in representing time

UT1 was incorrectly specified as our time representation.  (UT1 is
astronomical time.)  We are not actually UTC either because we ignore
leap seconds.

Reported-by: Thomas Munro

Discussion: CAEepm=3-TW9PLwGZhqjSSiEQ9UzJEKE-HELQDzRE0QUSCp8dgw@mail.gmail.com

doc/src/sgml/catalogs.sgml

@@ -9804,8 +9804,8 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
    of time zone names that are recognized by <command>SET TIMEZONE</>,
    along with their associated abbreviations, UTC offsets,
    and daylight-savings status.  (Technically,
-   <productname>PostgreSQL</productname> uses <acronym>UT1</> rather
-   than UTC because leap seconds are not handled.)
+   <productname>PostgreSQL</productname> does not use 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

@@ -7578,7 +7578,7 @@ SELECT EXTRACT(SECOND FROM TIME '17:12:28.5');
         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.  (Technically,
-        <productname>PostgreSQL</productname> uses <acronym>UT1</> because
+        <productname>PostgreSQL</productname> does not use UTC because
         leap seconds are not handled.)
        </para>
       </listitem>