Clarify IsNonNullPosition algorithm

Author: benjie

spec/Section 5 -- Validation.md

@@ -1906,11 +1906,11 @@ IsVariableUsageAllowed(variableDefinition, variableUsage):
 
 IsNonNullPosition(locationType, variableUsage):
 
-- Let {isOneOfField} be {true} if {variableUsage} is located within an
-  {ObjectField} entry and the parent type of {ObjectField} is a OneOf Input
-  Object; otherwise {false}.
-- If {isOneOfField} is {true} or {locationType} is a non-null type, return
-  {true}.
+- If {locationType} is a non-null type, return {true}.
+- If the location of {variableUsage} is an {ObjectField}:
+  - Let {parentLocationType} be the expected type of {ObjectField}'s parent
+    {ObjectValue}.
+  - If {parentLocationType} is a OneOf Input Object type, return {true}.
 - Return {false}.
 
 AreTypesCompatible(variableType, locationType):