Doc: Fix QSql*Model snippets

- QSqlQueryModel docs contained a snippet about QSqlTableModel.
- Snippet #25 was about QSqlTableModel, but it was previously unused.
- This patch ensures that snippet code matches the corresponding text
  descriptions.

Change-Id: I2a5ffbe0978ef9b8d0b027db59647b824e52d214
Reviewed-by: Samuel Gaist <samuel.gaist@idiap.ch>

src/sql/doc/snippets/sqldatabase/sqldatabase.cpp

@@ -249,16 +249,15 @@ void QSqlQueryModel_snippets()
     }
 
 //! [21]
-    QSqlTableModel model;
-    model.setTable("employee");
-    model.select();
+    QSqlQueryModel model;
+    model.setQuery("SELECT name, salary FROM employee");
     int salary = model.record(4).value("salary").toInt();
 //! [21]
     Q_UNUSED(salary);
 
     {
 //! [22]
-    int salary = model.data(model.index(4, 2)).toInt();
+    int salary = model.data(model.index(4, 1)).toInt();
 //! [22]
     Q_UNUSED(salary);
     }
@@ -308,7 +307,8 @@ void QSqlTableModel_snippets()
 //! [25]
     QSqlTableModel model;
     model.setTable("employee");
-    QString name = model.record(4).value("name").toString();
+    model.select();
+    int salary = model.record(4).value("salary").toInt();
 //! [25]
     }
 }

src/sql/models/qsqlquerymodel.cpp

@@ -124,8 +124,8 @@ int QSqlQueryModelPrivate::columnInQuery(int modelColumn) const
     \snippet sqldatabase/sqldatabase.cpp 21
 
     The code snippet above extracts the \c salary field from record 4 in
-    the result set of the query \c{SELECT * from employee}. Assuming
-    that \c salary is column 2, we can rewrite the last line as follows:
+    the result set of the \c SELECT query. Since \c salary is the 2nd
+    column (or column index 1), we can rewrite the last line as follows:
 
     \snippet sqldatabase/sqldatabase.cpp 22
 

src/sql/models/qsqltablemodel.cpp

@@ -222,7 +222,7 @@ bool QSqlTableModelPrivate::exec(const QString &stmt, bool prepStatement,
     QSqlTableModel can also be used to access a database
     programmatically, without binding it to a view:
 
-    \snippet sqldatabase/sqldatabase.cpp 21
+    \snippet sqldatabase/sqldatabase.cpp 25
 
     The code snippet above extracts the \c salary field from record 4 in
     the result set of the query \c{SELECT * from employee}.