Reword JWT config claim (#69356)

Co-authored-by: ptz-xyt <104002657+ptz-xyt@users.noreply.github.com>

docs/sources/setup-grafana/configure-security/configure-authentication/jwt/index.md

@@ -36,7 +36,7 @@ header_name = X-JWT-Assertion
 
 ## Configure login claim
 
-To identify the user, some of the claims needs to be selected as a login info. You could specify a claim that contains either a username or an email of the Grafana user.
+To identify the user, some of the claims needs to be selected as a login info. The subject claim called `"sub"` is mandatory and needs to identify the principal that is the subject of the JWT.
 
 Typically, the subject claim called `"sub"` would be used as a login but it might also be set to some application specific claim.