Update purchase order from sales order docs

[metas-Kay]

• http://docs.metasfresh.org/webui_collection/EN/Create_individual_PO_from_SO.html
• http://docs.metasfresh.org/webui_collection/EN/Set_default_vendor
• http://docs.metasfresh.org/webui_collection/EN/Link_product_to_business_partner

-> missing info: Vendor must have a discount schema set in order to show up in the purchase order disposition overlay

[metas-Kay]

• update also according to video: https://github.com/metasfresh/metasfresh-e2e/issues/145