feat: document login hints feature

docs/identities/sign-in/actions.mdx

@@ -131,6 +131,33 @@ created account to get access to your services, but instead can access all the f
 1. Go to Ory Console → [**Base Settings**](https://console.ory.sh/projects/current/auth).
 2. Toggle **Sign In After Registration**.
 
+## Provide login hints to users
+
+When users attempt to sign up with an identifier (username, email address etc.) which has previously been registered, they will be
+presented an error explaining that an account is already on file and advising them to login to that account.
+
+If you have a number of different sign-in methods configured (say username and password plus one or more social sign-in
+providers), your users might not remember with which provider they initially signed up.
+
+You can configure your Ory Network project to provide login hints to your users in this scenario. If a user attempts to sign up
+but an account already exists, the error message will tell them exactly which sign-in provider to use instead.
+
+To enable these login hints, use the Ory CLI:
+
+```shell
+ory patch project ${project_id} \
+  --replace '/services/identity/config/selfservice/flows/registration/login_hints=true'
+```
+
+:::info
+
+This behavior improves the sign-in experience for your users, but comes at the cost of exposing information about which sign-in
+mehods a particular account identifier has associated with it.
+
+Do not enable this feature if account enumeration attacks are a risk factor in your threat model.
+
+:::
+
 ## Show verification after successful registration
 
 If you want to show the verification screen after registration, follow these steps: