Document that to_char('J'/Julian) is midnight-based, per report that

bmomjian · bmomjian · commit c62de15becf0 · 2007-02-19T22:51:44.000Z
Julian technically is noon to noon.
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.364 2007/02/19 22:10:52 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.365 2007/02/19 22:51:44 momjian Exp $ -->
 
  <chapter id="functions">
   <title>Functions and Operators</title>
@@ -4641,7 +4641,7 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
        </row>
        <row>
         <entry><literal>J</literal></entry>
-        <entry>Julian Day (days since January 1, 4712 BC)</entry>
+        <entry>Julian Day (days since midnight, January 1, 4712 BC)</entry>
        </row>
        <row>
         <entry><literal>Q</literal></entry>
