Add definition of "selection set" and clarify serial execution examples

[benjie]

Examples 193 and 194 relate to executing selection sets serially. There has been confusion from readers that think that these examples relate to documents (i.e. they should have the mutation keyword) but that is not necessarily the case, for example in future we might enable operations such as:

mutation {
  ... @defer {
    mutation1 { result }
    mutation2 { result }
  }
  ... @defer {
    mutation3 { result }
    mutation4 { result }
  }
}

To solve this:

1. I've changed the first example to be a mutation operation and changed the leading text to state this explicitly
2. For the second example I've put a note inside the example making it clear once again that this is a selection set and not a query or mutation document
3. I've created a definition for "selection set"
4. I've updated various references to "selection set" throughout the document (though not in algorithms) to link to this definition

I've also fixed the consistency of the casing of 'selection set'.

• Fixes add note about selection sets in serial execution examples #926
• Fixes added mutation op type to examples 193, 194 #984

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[benjie]

🆕 Definition of selection set.

[benjie]

🆕 Clarification

[Keweiqu]

define variable

[Keweiqu]

Perhaps we add the mutation example, and keep the selection set example?

[benjie]

🆕 Note added

[mjmahone]

This seems like a positive editorial change

[benjie]

@Keweiqu I've addressed your feedback, I think? Very good catch! 🙌

[leebyron]

good catch