docs: more feedback

More feedback

docs/site/Req-res-cycle.md

@@ -74,7 +74,7 @@ they are capable of adding new endpoints and determining the result of a request
 ### The request-response cycle
 
 The request-response cycle involves many components, especially if the request
-is to a controller-endpoint which interacts with a model.
+is to a controller-endpoint which interacts with the database.
 
 ![Components of LoopBack 4 request-response cycle](./imgs/req-res-high-level.png)
 
@@ -111,6 +111,10 @@ route for invocation.
 parses LoopBack-relevant request paremeters from the request body, URL segment,
 and query parameters.
 
+It is also responsible for validating the property types of the request body,
+as defined in the model. If the request body does not match the schema, it
+throws a HTTP 422 error.
+
 ##### 3. InvokeMethod
 
 [InvokeMethod](https://loopback.io/doc/en/lb4/apidocs.rest.invokemethodprovider.html)
@@ -138,6 +142,44 @@ responsible for sending error back to the client in case any of the above helper
 methods throw or encounter any errors; this includes `4xx`, `5xx`, and any
 other type of errors.
 
+The implementation of the `handle()` method of the default sequence looks like
+this:
+
+```ts
+async handle(context: RequestContext) {
+  try {
+    const {request, response} = context;
+    const route = this.findRoute(request);
+    const args = await this.parseParams(request, route);
+    const result = await this.invoke(route, args);
+    this.send(response, result);
+  } catch (err) {
+    this.reject(context, err);
+  }
+}
+```
+
+Any provider can be plugged in in the sequence for additional functionality in
+the app. For example, the [authentication provider](https://loopback.io/doc/en/lb4/apidocs.authentication.authenticateactionprovider.html)
+may be plugged in right after the `FindRoute` phase to enforce authentication
+on all requests to the app.
+
+```ts
+export class MySequence implements SequenceHandler {
+  constructor(
+    ...
+    @inject(AuthenticationBindings.AUTH_ACTION)
+    protected authenticateRequest: AuthenticateFn,
+  ) {}
+
+  async handle(context: RequestContext) {
+    try {
+      const {request, response} = context;
+      const route = this.findRoute(request);
+      await this.authenticateRequest(request);
+...
+```
+
 #### Request to a controller endpoint
 
 Requests to a controller endpoint are handled in a very different context than
@@ -198,6 +240,10 @@ All controllers, services, and repositories; and their methods can be intercepte
 This makes interceptors a powerful participant in the request-response cycle,
 since they can modify the request and response objects and call their methods.
 
+{% include tip.html content="The [authorization component](https://loopback.io/doc/en/lb4/apidocs.authorization.html)
+is an example of the use of interceptors to add additional functionality to the
+app." %}
+
 ##### Walk through of a request
 
 Now that we know all the components that may be involve in a request to a