Improve description for @oneOf directive

Follow up from #3513.

I think it is fine for the spec text in graphql/graphql-spec#825
to refer to `OneOf Input Object`, as the spec explains
what that is in another section. In a directive definition, that explanation by itself
does not make much sense without prior knowledge.

Author: spawnia

src/type/directives.ts

@@ -248,11 +248,11 @@ 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 that exactly one field must be supplied and that field must not be `null`.',
   locations: [DirectiveLocation.INPUT_OBJECT],
   args: {},
 });

src/utilities/__tests__/printSchema-test.ts

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