Doc: fix out-of-date example of SPI usage.

The "count" argument of SPI_exec() only limits execution when
the query is actually returning rows.  This was not the case
before PG 9.0, so this example was correct when written; but
we missed updating it in commit 2ddc600f8.  Extend the example
to show the behavior both with and without RETURNING.

While here, improve the commentary and markup for the rest
of the example.

David G. Johnston and Tom Lane, per report from Curt Kolovson.
Back-patch to all supported branches.

Discussion: https://postgr.es/m/CANhYJV6HWtgz_qjx_APfK0PAgLUzY-2vjLuj7i_o=TZF1LAQew@mail.gmail.com

doc/src/sgml/spi.sgml

@@ -5314,15 +5314,16 @@ CREATE FUNCTION execq(text, integer) RETURNS int8
 => INSERT INTO a VALUES (execq('INSERT INTO a VALUES (0)', 0));
 INSERT 0 1
 => SELECT execq('SELECT * FROM a', 0);
-INFO:  EXECQ:  0    -- inserted by execq
-INFO:  EXECQ:  1    -- returned by execq and inserted by upper INSERT
+INFO:  EXECQ:  0    <lineannotation>-- inserted by execq</lineannotation>
+INFO:  EXECQ:  1    <lineannotation>-- returned by execq and inserted by upper INSERT</lineannotation>
 
  execq
 -------
      2
 (1 row)
 
-=&gt; SELECT execq('INSERT INTO a SELECT x + 2 FROM a', 1);
+=&gt; SELECT execq('INSERT INTO a SELECT x + 2 FROM a RETURNING *', 1);
+INFO:  EXECQ:  2    <lineannotation>-- 0 + 2, then execution was stopped by count</lineannotation>
  execq
 -------
      1
@@ -5331,21 +5332,38 @@ INFO:  EXECQ:  1    -- returned by execq and inserted by upper INSERT
 =&gt; SELECT execq('SELECT * FROM a', 10);
 INFO:  EXECQ:  0
 INFO:  EXECQ:  1
-INFO:  EXECQ:  2    -- 0 + 2, only one row inserted - as specified
+INFO:  EXECQ:  2
+
+ execq
+-------
+     3              <lineannotation>-- 10 is the max value only, 3 is the real number of rows</lineannotation>
+(1 row)
 
+=&gt; SELECT execq('INSERT INTO a SELECT x + 10 FROM a', 1);
  execq
 -------
-     3              -- 10 is the max value only, 3 is the real number of rows
+     3              <lineannotation>-- all rows processed; count does not stop it, because nothing is returned</lineannotation>
 (1 row)
 
+=&gt; SELECT * FROM a;
+ x
+----
+  0
+  1
+  2
+ 10
+ 11
+ 12
+(6 rows)
+
 =&gt; DELETE FROM a;
-DELETE 3
+DELETE 6
 =&gt; INSERT INTO a VALUES (execq('SELECT * FROM a', 0) + 1);
 INSERT 0 1
 =&gt; SELECT * FROM a;
  x
 ---
- 1                  -- no rows in a (0) + 1
+ 1                  <lineannotation>-- 0 (no rows in a) + 1</lineannotation>
 (1 row)
 
 =&gt; INSERT INTO a VALUES (execq('SELECT * FROM a', 0) + 1);
@@ -5355,15 +5373,16 @@ INSERT 0 1
  x
 ---
  1
- 2                  -- there was one row in a + 1
+ 2                  <lineannotation>-- 1 (there was one row in a) + 1</lineannotation>
 (2 rows)
 
--- This demonstrates the data changes visibility rule:
+<lineannotation>-- This demonstrates the data changes visibility rule.</lineannotation>
+<lineannotation>-- execq is called twice and sees different numbers of rows each time:</lineannotation>
 
 =&gt; INSERT INTO a SELECT execq('SELECT * FROM a', 0) * x FROM a;
-INFO:  EXECQ:  1
+INFO:  EXECQ:  1    <lineannotation>-- results from first execq</lineannotation>
 INFO:  EXECQ:  2
-INFO:  EXECQ:  1
+INFO:  EXECQ:  1    <lineannotation>-- results from second execq</lineannotation>
 INFO:  EXECQ:  2
 INFO:  EXECQ:  2
 INSERT 0 2
@@ -5372,10 +5391,9 @@ INSERT 0 2
 ---
  1
  2
- 2                  -- 2 rows * 1 (x in first row)
- 6                  -- 3 rows (2 + 1 just inserted) * 2 (x in second row)
-(4 rows)               ^^^^^^
-                       rows visible to execq() in different invocations
+ 2                  <lineannotation>-- 2 rows * 1 (x in first row)</lineannotation>
+ 6                  <lineannotation>-- 3 rows (2 + 1 just inserted) * 2 (x in second row)</lineannotation>
+(4 rows)
 </programlisting>
   </para>
  </sect1>