Add docs about `fieldName:altFieldName' syntax

[mattbrailsford]

When using the HTML API, the value attributes for things like <input name="SetCurrentShippingMethod" value="shippingMethodId" type="hidden"> are hard coded, but you can change where TC look with the syntax <input name="SetCurrentShippingMethod" value="shippingMethodId:altFieldName" type="hidden"> but this isn't documented.

[mattbrailsford]

Docs do exist here https://docs.teacommerce.net/3.3.0/api/#multiple-methods, but I don't think this is clear enough that you can also use this snytax as a way to just define the form field name you want to use over the default.