[Tuple specification] The term "exact match" rule is not defined

[BillWagner]

see #3031 (comment)

The proposal uses the term "exact match" for tuples that have the same arity and the same member types. This needs to be defined, or this rule needs to be restated using existing rules.

[BillWagner]

This was defined as an identity conversion in the C# 7 standard