docs(webhook-receivers.md): validation

fix fluxcd/notification-controller#256

references:
https://fluxcd.io/docs/components/notification/receiver/
https://github.com/fluxcd/notification-controller/blob/main/internal/server/receiver_handlers.go#L123
https://github.com/fluxcd/notification-controller/blob/main/internal/server/receiver_handlers.go#L135
https://github.com/google/go-github/blob/v39.2.0/github/messages.go#L145:L254
https://github.com/google/go-github/blob/v39.2.0/github/messages.go#L109:L113

Signed-off-by: lloydchang <lloydchang@gmail.com>

content/en/docs/guides/webhook-receivers.md

@@ -136,6 +136,9 @@ spec:
       name: webapp
 ```
 
+* When the receiver type is set to generic, the controller will not perform token validation nor event filtering
+* Generic receiver's payload content is not validated. Only the signature is verified using HMAC
+
 {{% alert color="info" title="Other receiver" %}}
 Besides GitHub, you can define receivers for **GitLab**, **Bitbucket**, **Harbor**
 and any other system that supports webhooks e.g. Jenkins, CircleCI, etc.
@@ -162,7 +165,7 @@ Fill the form with:
 With the above settings, when you push a commit to the repository, the following happens:
 
 * GitHub sends the Git push event to the receiver address
-* Notification controller validates the authenticity of the payload using HMAC
+* Notification controller validates an incoming GitHub Webhook event request's signature using HMAC and returns the (JSON) payload
 * Source controller is notified about the changes
 * Source controller pulls the changes into the cluster and updates the `GitRepository` revision
 * Kustomize controller is notified about the revision change