Merge branch '18256-secret-token-docs' into 'master'

Document that webhook secret token is sent in X-Gitlab-Token HTTP header

## What does this MR do?

Note that the secret token is sent in the X-Gitlab-Token header on the webhook documentation page, as well as directly below the secret token field on the webhook settings form.

## Are there points in the code the reviewer needs to double check?

No.

## Why was this MR needed?

It took me a while to figure out how to verify the token in my hook endpoint. Issue #18256 is where I found how to do it.

## What are the relevant issue numbers?

#18256 

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
Closes #18256

See merge request !5664

CHANGELOG

@@ -39,6 +39,7 @@ v 8.11.0 (unreleased)
   - Include old revision in merge request update hooks (Ben Boeckel)
   - Add build event color in HipChat messages (David Eisner)
   - Make fork counter always clickable. !5463 (winniehell)
+  - Document that webhook secret token is sent in X-Gitlab-Token HTTP header !5664 (lycoperdon)
   - Gitlab::Highlight is now instrumented
   - All created issues, API or WebUI, can be submitted to Akismet for spam check !5333
   - The overhead of instrumented method calls has been reduced

app/views/shared/web_hooks/_form.html.haml

@@ -19,7 +19,7 @@
         = f.label :token, "Secret Token", class: 'label-light'
         = f.text_field :token, class: "form-control", placeholder: ''
         %p.help-block
-          Use this token to validate received payloads
+          Use this token to validate received payloads. It will be sent with the request in the X-Gitlab-Token HTTP header.
       .form-group
         = f.label :url, "Trigger", class: 'label-light'
         %ul.list-unstyled

doc/web_hooks/web_hooks.md

@@ -26,6 +26,10 @@ GitLab webhooks keep in mind the following things:
     you are writing a low-level hook this is important to remember.
 -   GitLab ignores the HTTP status code returned by your endpoint.
 
+## Secret Token
+
+If you specify a secret token, it will be sent with the hook request in the `X-Gitlab-Token` HTTP header. Your webhook endpoint can check that to verify that the request is legitimate.
+
 ## SSL Verification
 
 By default, the SSL certificate of the webhook endpoint is verified based on