Doc: improve explanation of EXTRACT(EPOCH) for timestamp without tz.

Try to be clearer about what computation is actually happening here.

Per bug #16797 from Dana Burd.

Discussion: https://postgr.es/m/16797-f264b0b980b53b8b@postgresql.org

doc/src/sgml/func.sgml

@@ -9354,9 +9354,11 @@ SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40');
       <listitem>
        <para>
         For <type>timestamp with time zone</type> values, the
-        number of seconds since 1970-01-01 00:00:00 UTC (can be negative);
+        number of seconds since 1970-01-01 00:00:00 UTC (negative for
+        timestamps before that);
         for <type>date</type> and <type>timestamp</type> values, the
-        number of seconds since 1970-01-01 00:00:00 local time;
+        nominal number of seconds since 1970-01-01 00:00:00,
+        without regard to timezone or daylight-savings rules;
         for <type>interval</type> values, the total number
         of seconds in the interval
        </para>
@@ -9365,18 +9367,29 @@ SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40');
 SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40.12-08');
 <lineannotation>Result: </lineannotation><computeroutput>982384720.12</computeroutput>
 
+SELECT EXTRACT(EPOCH FROM TIMESTAMP '2001-02-16 20:38:40.12');
+<lineannotation>Result: </lineannotation><computeroutput>982355920.12</computeroutput>
+
 SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours');
 <lineannotation>Result: </lineannotation><computeroutput>442800</computeroutput>
 </screen>
 
        <para>
-        You can convert an epoch value back to a time stamp
+        You can convert an epoch value back to a <type>timestamp with time zone</type>
         with <function>to_timestamp</function>:
        </para>
 <screen>
 SELECT to_timestamp(982384720.12);
 <lineannotation>Result: </lineannotation><computeroutput>2001-02-17 04:38:40.12+00</computeroutput>
 </screen>
+
+       <para>
+        Beware that applying <function>to_timestamp</function> to an epoch
+        extracted from a <type>date</type> or <type>timestamp</type> value
+        could produce a misleading result: the result will effectively
+        assume that the original value had been given in UTC, which might
+        not be the case.
+       </para>
       </listitem>
      </varlistentry>