userid format: clarify that this is the full name@realm in description

as it recently confused a user in the forum.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/PVE/Auth/Plugin.pm

@@ -127,7 +127,7 @@ sub verify_username {
 }
 
 PVE::JSONSchema::register_standard_option('userid', {
-    description => "User ID",
+    description => "Full User ID, in the `name\@realm` format.",
     type => 'string', format => 'pve-userid',
     maxLength => 64,
 });