Allow recursion within ResolveAbstractType

This formalizes the proprosed feature within `graphql-js` whereby the internal method provided by JavaScript for runtime type resolution is allowed to return an intermediate interface.

See:
Issue: graphql/graphql-js#3253
PR: graphql/graphql-js#3599

@IvanGoncharov [suggested](graphql/graphql-js#3599 (review)) that this would require a spec change. Alternatively, perhaps the recursion [should be considered to be a feature of  the internal system](graphql/graphql-js#3599 (comment)) itself, possibly limited to JavaScript-like implementations.

This PR provides some potential spec text, were a spec change to be considered necessary.

Author: yaacovCR

spec/Section 6 -- Execution.md

@@ -722,18 +722,27 @@ and output of {CoerceResult()} must not be {null}.
 
 When completing a field with an abstract return type, that is an Interface or
 Union return type, first the abstract type must be resolved to a relevant Object
-type. This determination is made by the internal system using whatever means
-appropriate.
+type. The internal system must provided a method for doing so using whatever
+means appropriate.
+
+Note: In cases where interfaces may implement interfaces, creating an abstract
+type hierarchy, the provided internal method may find it convenient on some
+systems to return an intermediate Interface type rather than the runtime Object
+type, requiring recursion, as detailed below.
 
 Note: A common method of determining the Object type for an {objectValue} in
 object-oriented environments, such as Java or C#, is to use the class name of
 the {objectValue}.
 
 ResolveAbstractType(abstractType, objectValue):
 
-- Return the result of calling the internal method provided by the type system
-  for determining the Object type of {abstractType} given the value
+- Let {possibleRuntimeType} be the result of calling the internal method
+  provided by the type system, given Abstract type {abstractType} and value
   {objectValue}.
+- If {possibleRuntimeType} is an Object type:
+  - Return {possibleRuntimeType}.
+- If {possibleRuntimeType} is an Interface type:
+  - Return ResolveAbstractTypeImpl(possibleRuntimeType, objectValue).
 
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