Better justify why subsitution produces `MINUS_NULL` instead of `NO_CHANGE`.
Better justify why subsitution produces `MINUS_NULL` instead of `NO_CHANGE`.

I think _this_ reason actually makes sense. I had previously concluded
that it "didn't matter" because it arises only in the case of code that
is otherwise incorrect. But given that such code can arise through
reasonable usages of legacy APIs (e.g.,
`someFluentIterable.filter(someClass::isInstance).first()`), it seems
worth mentioning.

jspecify/jspecify