Fix SameResponseShape definition (#451)

* Fix `SameResponseShape` definition

* Capitalize first letter

* Address review

* Address review v2

Author: matprec

spec/Section 5 -- Validation.md

@@ -421,23 +421,25 @@ SameResponseShape(fieldA, fieldB) :
   * Let {typeA} be the return type of {fieldA}.
   * Let {typeB} be the return type of {fieldB}.
   * If {typeA} or {typeB} is Non-Null.
-    * {typeA} and {typeB} must both be Non-Null.
+    * If {typeA} or {typeB} is nullable, return false.
     * Let {typeA} be the nullable type of {typeA}
     * Let {typeB} be the nullable type of {typeB}
   * If {typeA} or {typeB} is List.
-    * {typeA} and {typeB} must both be List.
+    * If {typeA} or {typeB} is not List, return false.
     * Let {typeA} be the item type of {typeA}
     * Let {typeB} be the item type of {typeB}
     * Repeat from step 3.
   * If {typeA} or {typeB} is Scalar or Enum.
-    * {typeA} and {typeB} must be the same type.
-  * Assert: {typeA} and {typeB} are both composite types.
+    * If {typeA} and {typeB} are the same type return true, otherwise return
+      false.
+  * If {typeA} or {typeB} is not a composite type, return false.
   * Let {mergedSet} be the result of adding the selection set of {fieldA} and
     the selection set of {fieldB}.
   * Let {fieldsForName} be the set of selections with a given response name in
     {mergedSet} including visiting fragments and inline fragments.
   * Given each pair of members {subfieldA} and {subfieldB} in {fieldsForName}:
-    * {SameResponseShape(subfieldA, subfieldB)} must be true.
+    * If {SameResponseShape(subfieldA, subfieldB)} is false, return false.
+  * Return true.
 
 **Explanatory Text**