Clarify that selection sets cannot be empty

[benjie]

This paragraph is trying to make clear that if you have a type like type Foo { foo: Int!, bar: Int!, baz: Int! } that the selection set { foo bar } is valid for it - you do not have to select baz even if it's non-nullable, since non-nullable does not mean "required" for output fields.

However, it does read ambiguously, implying that you may remove any field from any selection set and the selection set will still be valid. If you remove the field foo from the selection set { foo } then that selection set ({ }) is no longer valid, since it is empty.

I've added a clarifying parenthesis to the text.

Fixes #1024

@graphql/tsc Can I get a couple checkmarks please? 😉

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