docs: add more docs about app.context

references:

- https://github.com/koajs/koa/issues/652

docs/api/index.md

@@ -184,13 +184,27 @@ ctx.cookies.set('name', 'tobi', { signed: true });
 
 ## app.context
 
-  The recommended namespace to extend with information that's useful
-  throughout the lifetime of your application, as opposed to a per request basis.
+  `app.context` is the prototype from which `ctx` is created from. 
+  You may add additional properties to `ctx` by editing `app.context`.
+  This is useful for adding properties or methods to `ctx` to be used across your entire app,
+  which may be more performant (no middleware) and/or easier (fewer `require()`s)
+  at the expense of relying more on `ctx`, which could be considered an anti-pattern.
+
+  For example, to add a reference to your database from `ctx`:
 
 ```js
 app.context.db = db();
+
+app.use(async (ctx) => {
+  console.log(ctx.db);
+});
 ```
 
+Note:
+
+- Many properties on `ctx` are defined using getters, setters, and `Object.defineProperty()`. You can only edit these properties (not recommended) by using `Object.defineProperty()` on `app.context`. See https://github.com/koajs/koa/issues/652.
+- Mounted apps currently use its parent's `ctx` and settings. Thus, mounted apps are really just groups of middleware.
+
 ## Error Handling
 
   By default outputs all errors to stderr unless `app.silent` is `true`.