Confusing language in Intersect logical specification

[mattwiller]

In the Intersect logical spec, the expression is marked as descending from NaryExpression. However, the verbiage below refers to "both arguments" and "either argument", making it sound like Intersect is actually a BinaryExpression. Which is correct? Some other expressions like Union and Except also have the same issue; I'd be happy to put together a PR to align the language in all of them, once I know which way to edit!