Reword explanation of rowCount (php#1843)

reference/pdo/pdostatement/rowcount.xml

@@ -20,9 +20,10 @@
    executed by the corresponding <literal>PDOStatement</literal> object. 
   </para>
   <para>
-   If the last SQL statement executed by the associated 
-   <literal>PDOStatement</literal> was a SELECT statement, some databases
-   may return the number of rows returned by that statement. However, this
+   For statements that produce result sets, such as <literal>SELECT</literal>,
+   the behavior is undefined and can be different for each driver.
+   Some databases may return the number of rows produced by that statement
+   (e.g. MySQL in buffered mode), but this
    behaviour is not guaranteed for all databases and should not be relied
    on for portable applications.
   </para>