Add missed reverse proxy authentication documentation (#22250)

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>
Co-authored-by: Jason Song <i@wolfogre.com>

docs/content/doc/features/authentication.en-us.md

@@ -329,3 +329,22 @@ Before activating SSPI single sign-on authentication (SSO) you have to prepare y
   - You have added the URL of the web app to the `Local intranet zone`
   - The clocks of the server and client should not differ with more than 5 minutes (depends on group policy)
   - `Integrated Windows Authentication` should be enabled in Internet Explorer (under `Advanced settings`)
+
+## Reverse Proxy
+
+Gitea supports Reverse Proxy Header authentication, it will read headers as a trusted login user name or user email address. This hasn't been enabled by default, you can enable it with
+
+```ini
+[service]
+ENABLE_REVERSE_PROXY_AUTHENTICATION = true
+```
+
+The default login user name is in the `X-WEBAUTH-USER` header, you can change it via changing `REVERSE_PROXY_AUTHENTICATION_USER` in app.ini. If the user doesn't exist, you can enable automatic registration with `ENABLE_REVERSE_PROXY_AUTO_REGISTRATION=true`.
+
+The default login user email is `X-WEBAUTH-EMAIL`, you can change it via changing `REVERSE_PROXY_AUTHENTICATION_EMAIL` in app.ini, this could also be disabled with `ENABLE_REVERSE_PROXY_EMAIL`
+
+If set `ENABLE_REVERSE_PROXY_FULL_NAME=true`, a user full name expected in `X-WEBAUTH-FULLNAME` will be assigned to the user when auto creating the user. You can also change the header name with `REVERSE_PROXY_AUTHENTICATION_FULL_NAME`.
+
+You can also limit the reverse proxy's IP address range with `REVERSE_PROXY_TRUSTED_PROXIES` which default value is `127.0.0.0/8,::1/128`. By `REVERSE_PROXY_LIMIT`, you can limit trusted proxies level.
+
+Notice: Reverse Proxy Auth doesn't support the API. You still need an access token or basic auth to make API requests.