docs: add clarification on user ID usage in MetadataLookup

- Added a comment in the MetadataLookup method to clarify that the user ID serves as the unique identifier for users across the system, enhancing code readability and understanding.

Jira Ticket: https://linuxfoundation.atlassian.net/browse/LFXV2-588

Signed-off-by: Mauricio Zanetti Salomao <mauriciozanetti86@gmail.com>

Author: mauriciozanettisalomao

internal/infrastructure/authelia/user.go

@@ -146,6 +146,7 @@ func (u *userReaderWriter) MetadataLookup(ctx context.Context, input string, use
 			return false
 		}
 		user.Sub = sub.String()
+		// user ID is used across the system as the unique identifier for the user
 		user.UserID = sub.String()
 		return true
 	}