Update book.

docs/en/src/apollo_federation.md

@@ -8,7 +8,9 @@
 
 - The `external` property declares that a field comes from another service。
 
-- The `provides` property indicates the fields provided by a service. 
+- The `provides` directive is used to annotate the expected returned fieldset from a field on a base type that is guaranteed to be selectable by the gateway. 
+
+- The `requires` directive is used to annotate the required input fieldset from a base type for a resolver. It is used to develop a query plan where the required fields may not be needed by the client, but the service may need additional information from other services.
 
 ## Entity lookup function
 

docs/zh-CN/src/apollo_federation.md

@@ -10,7 +10,7 @@
 
 - 字段的`provides`属性用于要求网关提供的字段集。
 
-- 字段的`provides`属性表示解析该字段值需要依赖该类型的字段集。
+- 字段的`requires`属性表示解析该字段值需要依赖该类型的字段集。
 
 ## 实体查找函数