[docs] QLatin1String: add some guidance as to when to use it

Change-Id: I391be8bda3a5cb4873b89b437d2b76b1cd88261f
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Mitch Curtis <mitch.curtis@theqtcompany.com>