Add documentation about total_sum parameter for Order API + mention it in shipping fee calculation

[vmattila]

Checkout HTTP Form API already supports total_sum parameter for shipping fee calculations. Same support is available in Create/Modify/Get Order but yet undocumented.

See

• http://logitrail-integration.readthedocs.io/en/latest/checkout/http_form_api.html#new-order-fields
• Add support for total_sum parameter in Checkout HTTP Form API Codaone/woocommerce_logitrail_module#8