Update doc for setting `macaroon_secret_key` (#13443)

* Update doc for setting `macaroon_secret_key`

* newsfile

changelog.d/13443.doc

@@ -0,0 +1 @@
+Update documentation for config setting `macaroon_secret_key`.

docs/usage/configuration/config_documentation.md

@@ -2495,9 +2495,13 @@ track_appservice_user_ips: true
 ---
 ### `macaroon_secret_key`
 
-A secret which is used to sign access tokens. If none is specified,
-the `registration_shared_secret` is used, if one is given; otherwise,
-a secret key is derived from the signing key.
+A secret which is used to sign
+- access token for guest users,
+- short-term login token used during SSO logins (OIDC or SAML2) and
+- token used for unsubscribing from email notifications.
+
+If none is specified, the `registration_shared_secret` is used, if one is given;
+otherwise, a secret key is derived from the signing key.
 
 Example configuration:
 ```yaml