fix: Add comments for re-writing properties from Koa (#3332)

index.d.ts

@@ -710,13 +710,20 @@ declare module 'egg' {
   * the egg's Context interface, which is wrong here because we have our own
   * special properties (e.g: encrypted). So we must remove this property and
   * create our own with the same name.
+  * @see https://github.com/eggjs/egg/pull/2958
+  * 
+  * However, the latest version of Koa has "[key: string]: any" on the
+  * context, and there'll be a type error for "keyof koa.Context".
+  * So we have to directly inherit from "KoaApplication.BaseContext" and
+  * rewrite all the properties to be compatible with types in Koa.
+  * @see https://github.com/eggjs/egg/pull/3329
   */
   export interface Context extends KoaApplication.BaseContext {
     [key: string]: any;
 
     app: Application;
 
-    // property of koa.Context
+    // properties of koa.Context
     req: IncomingMessage;
     res: ServerResponse;
     originalUrl: string;
@@ -789,8 +796,7 @@ declare module 'egg' {
     realStatus: number;
 
     /**
-     * 设置返回资源对象
-     * set the ctx.body.data value
+     * Set the ctx.body.data value
      *
      * @member {Object} Context#data=
      * @example