Add note about external_ids for User Admin API in documentation

Signed-off-by: Jacob Sánchez <jacobszpz@protonmail.com>

docs/admin_api/user_admin_api.md

@@ -141,8 +141,8 @@ Body parameters:
   provider for SSO (Single sign-on). More details are in the configuration manual under the
   sections [sso](../usage/configuration/config_documentation.md#sso) and [oidc_providers](../usage/configuration/config_documentation.md#oidc_providers).
   - `auth_provider` - **string**, required. The unique, internal ID of the external identity provider.
-    The same as `idp_id` from the homeserver configuration. Note that no error is raised if the
-    provided value is not in the homeserver configuration.
+    The same as `idp_id` from the homeserver configuration. If using OIDC, this value should be prefixed
+    with `oidc-`. Note that no error is raised if the provided value is not in the homeserver configuration.
   - `external_id` - **string**, required. An identifier for the user in the external identity provider.
     When the user logs in to the identity provider, this must be the unique ID that they map to.
 - `admin` - **bool**, optional, defaults to `false`. Whether the user is a homeserver administrator,