Add some clarification to expiry in node schema

[kezz]

See example on petstore: https://petstore.swagger.io/?url=https://raw.githubusercontent.com/LuckPerms/rest-api/5a517a29835838dd63fe7f0518b5ae39e61e0e16/src/main/resources/luckperms-openapi.yml#/Users/add-user-node

[lucko]

Thanks :)