Update @shareable rules

[glen-84]

#147 (comment)

Based on an internal discussion:

1. We'll move this to a pre-merge validation rule.
2. We'll move the "subscription root fields cannot be shared" part to INVALID_SHAREABLE_USAGE.
3. We'll update "excluding fields marked with @external or @override" to clarify that it's the overridden field, not the overriding field, that is excluded.
4. We'll update the spec to require all source schemas to mark the field as shareable, not just one of them.
5. We'll simplify the counter-example.

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