Servers must preserve lexical ordering between SDL and introspection

[spawnia]

Resolves #577

This change adds the proposed addition by @leebyron from #577 (comment)

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[benjie]

I approve of the intent behind this. It doesn't feel like putting the current proposed edit in chapter 4 is quite right though. We have effectively this global rule:

The order of fields, arguments, etc in a GraphQL schema is significant

Then that comes with two corollaries:

1. The order of fields, arguments, etc in SDL is significant - when building a schema from SDL these orders should be maintained.
2. The order of fields, arguments, etc in introspection is significant - when introspecting the schema these orders should be maintained.

(1) Should be in section 3 ("Type System"), whereas (2) should be in section 4 ("Introspection"). I think (2) should come under each of the Fields: prompts, e.g.

• fields must return the set of fields that can be selected for this type.
  • Accepts the argument includeDeprecated which defaults to false. If true, deprecated fields are also returned.
  • Fields must be returned in the order in which they are defined in the schema

Similar localised edits should be made in section 3 I think. I'm not sure where the global rule belongs exactly, it could be that section 3 covers it.