Document behaviour of disposition undeliverable-here=true when auto-links are in use

Documentation ought to point out the behaviour of disposition undeliverable-here=true when auto links are in use. A user familiar with [1] would expect that such a disposition would make the message eligible all other competing consumers, however they are connected. When autolinks are in use and those competing consumers are attached to the same router, that won't be case.

[1] http://docs.oasis-open.org/amqp/core/v1.0/os/amqp-core-messaging-v1.0-os.html#type-modified