Replace ExecuteSelectionSet with ExecuteGroupedFieldSet

spec/Section 2 -- Language.md

@@ -466,8 +466,9 @@ These two operations are semantically identical:
 
 Alias : Name :
 
-By default a field's response key in the response object will use that field's
-name. However, you can define a different response key by specifying an alias.
+:: A _response key_ is the key in the response object which correlates with a
+field's result. By default the response key will use the field's name; however,
+you can define a different response key by specifying an alias.
 
 In this example, we can fetch two profile pictures of different sizes and ensure
 the resulting response object will not have duplicate keys:

spec/Section 5 -- Validation.md

@@ -468,7 +468,7 @@ unambiguous. Therefore any two field selections which might both be encountered
 for the same object are only valid if they are equivalent.
 
 During execution, the simultaneous execution of fields with the same response
-name is accomplished by {MergeSelectionSets()} and {CollectFields()}.
+name is accomplished by {CollectSubfields()}.
 
 For simple hand-written GraphQL, this rule is obviously a clear developer error,
 however nested fragments can make this difficult to detect manually.