doc: Additional documentation for date_bin

Reported-by: Justin Pryzby <pryzby@telsasoft.com>
Author: John Naylor <john.naylor@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/CAFBsxsEEm1nuhZmfVQxvu_i3nDDEuvNJ_WMrDo9whFD_jusp-A@mail.gmail.com

doc/src/sgml/func.sgml

@@ -9941,6 +9941,20 @@ SELECT date_trunc('hour', INTERVAL '3 days 02:47:33');
     aligned with a specified origin.
    </para>
 
+   <para>
+<synopsis>
+date_trunc(<replaceable>stride</replaceable>, <replaceable>source</replaceable>, <replaceable>origin</replaceable>)
+</synopsis>
+    <replaceable>source</replaceable> is a value expression of type
+    <type>timestamp</type> or <type>timestamp with time zone</type>.  (Values
+    of type <type>date</type> are cast automatically to
+    <type>timestamp</type>.)  <replaceable>stride</replaceable> is a value
+    expression of type <type> interval</type>.  The return value is likewise
+    of type <type>timestamp</type> or <type>timestamp with time zone</type>,
+    and it marks the beginning of the bin into which the
+    <replaceable>source</replaceable> is placed.
+   </para>
+
    <para>
     Examples:
 <screen>
@@ -9958,6 +9972,10 @@ SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-0
     that <function>date_bin</function> can truncate to an arbitrary interval.
    </para>
 
+   <para>
+    Negative intervals are allowed and are treated the same as positive intervals.
+   </para>
+
    <para>
     The <parameter>stride</parameter> interval cannot contain units of month
     or larger.