Introspection: Clarify nullability of fields in __Type type (#536)

Motivation: graphql/graphiql#746 (comment)

spec/Section 4 -- Introspection.md

@@ -129,22 +129,22 @@ type __Type {
   name: String
   description: String
 
-  # OBJECT and INTERFACE only
+  # should be non-null for OBJECT and INTERFACE only, must be null for the others
   fields(includeDeprecated: Boolean = false): [__Field!]
 
-  # OBJECT only
+  # should be non-null for OBJECT only, must be null for the others
   interfaces: [__Type!]
 
-  # INTERFACE and UNION only
+  # should be non-null for INTERFACE and UNION only, always null for the others
   possibleTypes: [__Type!]
 
-  # ENUM only
+  # should be non-null for ENUM only, must be null for the others
   enumValues(includeDeprecated: Boolean = false): [__EnumValue!]
 
-  # INPUT_OBJECT only
+  # should be non-null for INPUT_OBJECT only, must be null for the others
   inputFields: [__InputValue!]
 
-  # NON_NULL and LIST only
+  # should be non-null for NON_NULL and LIST only, must be null for the others
   ofType: __Type
 }