Add kind Row to Prim's Docs #3459

The best reference we have to a definition of Row is Record which is inaccurate because Rows are used in so many other places besides Records (e.g. Variant, type-level programming). I'd like to submit a PR that adds docs for it, but before I do, I want to discuss the best rendering for it. Here's my current rendering for Row:

Row is a kind for 0 or more label-type associations where the total number of these associations is known at compile time. A label can be used to refer to an instance of the associated type.

First, a row can be empty (there are no associations) or non-empty (there is at least one association). Second, it can be closed (the row contains only the label-type associations defined here) or open (the row contains at least, though it may contain more than, the label-type associations defined here). Third, it can contain duplicate 'labels'.

Examples:
Empty closed Row: ()
Empty open Row: ( | otherLabelTypeAssociations)
Non-empty closed Row: (label1 :: TypeName1, ... , labelN :: TypeNameN)
Non-empty open Row: (label1 :: TypeName1, ... , labelN :: TypeNameN | otherLabelTypeAssociations)
Duplicate Labels: (someLabel :: String, someLabel :: Int)

One open question I have. While rows can contain duplicate labels, can those labels associate different types? I'm assuming yes, but I'd like it clarified.