add max limit delay in documentation

[Pchol]

I have error when try delay 30 days in AMQP Interop driver and enqueue/amqp-bunny transport and rabbit with installed RabbitMQ Delayed Message Plugin. After inspect code and documentation i discover what rabbitmq delayed message plugin max delay is 2^32 milliseconds (~50 days) and bunny have limit for headers 2^31 milliseconds (~25 days)