Fix a Oracle-compatible instr function in the documentation

[Yugo Nagata]

Hi,

Attached is a patch to fix a very trivial issue of the documentation.

The documentation of PL/pgSQL provides sample codes of Oracle-compatible
instr functions. However, the behaviour is a little differet.
Oracle's instr raises an error when the forth argument value is less than
zero, but the sample code returns zero. This patch fixes this.

Regards,

-- 
Yugo Nagata <nag...@sraoss.co.jp>

diff --git a/doc/src/sgml/plpgsql.sgml b/doc/src/sgml/plpgsql.sgml
index 7d23ed4..5a67c38 100644
--- a/doc/src/sgml/plpgsql.sgml
+++ b/doc/src/sgml/plpgsql.sgml
@@ -5676,6 +5676,10 @@ DECLARE
     length integer;
     ss_length integer;
 BEGIN
+    IF occur_index &lt;= 0 THEN
+        RAISE 'argument ''%'' is out of range', occur_index USING ERRCODE = '22003';
+    END IF;
+
     IF beg_index &gt; 0 THEN
         temp_str := substring(string FROM beg_index);
         pos := position(string_to_search IN temp_str);