Improve description for @oneOf directive (graphql#3937)

Author: spawnia

src/type/directives.ts

@@ -248,11 +248,12 @@ export const GraphQLSpecifiedByDirective: GraphQLDirective =
   });
 
 /**
- * Used to declare an Input Object as a OneOf Input Objects.
+ * Used to indicate an Input Object is a OneOf Input Object.
  */
 export const GraphQLOneOfDirective: GraphQLDirective = new GraphQLDirective({
   name: 'oneOf',
-  description: 'Indicates an Input Object is a OneOf Input Object.',
+  description:
+    'Indicates exactly one field must be supplied and this field must not be `null`.',
   locations: [DirectiveLocation.INPUT_OBJECT],
   args: {},
 });

src/utilities/__tests__/printSchema-test.ts

@@ -795,7 +795,9 @@ describe('Type System Printer', () => {
         url: String!
       ) on SCALAR
 
-      """Indicates an Input Object is a OneOf Input Object."""
+      """
+      Indicates exactly one field must be supplied and this field must not be \`null\`.
+      """
       directive @oneOf on INPUT_OBJECT
 
       """