Adds a note to QSqlDatabase::~QSqlDatabase documentation

When the last QSqlDatabase object is destroyed the destructor implicitly
calls close() to release the database connection.

Task-number: QTBUG-59919
Change-Id: I04c15c4999cdaaa8800a44a1a1006f977a90d8a6
Reviewed-by: Samuel Gaist <samuel.gaist@edeltech.ch>
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

src/sql/kernel/qsqldatabase.cpp

@@ -695,6 +695,9 @@ void QSqlDatabasePrivate::init(const QString &type)
 /*!
     Destroys the object and frees any allocated resources.
 
+    \note When the last connection is destroyed, the destructor
+    implicitly calls close() to release the database connection.
+
     \sa close()
 */