Documentation for explicitly mentioning ObjectTypes (#1679)

* Documentation for explicitly mentioning ObjectTypes

Based on the discussion at #1413

* Minor typo/fix

* Edit example doc for schema definition

- provide more clear example /w interfaces & types
- rewrite to note with more clarity
- move the note above the note on directives

* clarify documentation

* Remove unwanted star

Author: elevenpassin

src/type/schema.js

@@ -72,6 +72,43 @@ export function assertSchema(schema: mixed): GraphQLSchema {
  *       mutation: MyAppMutationRootType,
  *     })
  *
+ * Note: When the schema is constructed, by default only the types that are
+ * reachable by traversing the root types are included, other types must be
+ * explicitly referenced.
+ *
+ * Example:
+ *
+ *     const characterInterface = new GraphQLInterfaceType({
+ *       name: 'Character',
+ *       ...
+ *     });
+ *
+ *     const humanType = new GraphQLObjectType({
+ *       name: 'Human',
+ *       interfaces: [characterInterface],
+ *       ...
+ *     });
+ *
+ *     const droidType = new GraphQLObjectType({
+ *       name: 'Droid',
+ *       interfaces: [characterInterface],
+ *       ...
+ *     });
+ *
+ *     const schema = new GraphQLSchema({
+ *       query: new GraphQLObjectType({
+ *         name: 'Query',
+ *         fields: {
+ *           hero: { type: characterInterface, ... },
+ *         }
+ *       }),
+ *       ...
+ *       // Since this schema references only the `Character` interface it's
+ *       // necessary to explicitly list the types that implement it if
+ *       // you want them to be included in the final schema.
+ *       types: [humanType, droidType],
+ *     })
+ *
  * Note: If an array of `directives` are provided to GraphQLSchema, that will be
  * the exact list of directives represented and allowed. If `directives` is not
  * provided then a default set of the specified directives (e.g. @include and