Doc mandatory param; cross link to set_return_handler

[bitdancer]

I spent a while digging around in the source before I found that one gets to handle unpublishable messages by setting a return handler on the channel. A link from publish to that method would have saved me a lot of time :)

[coveralls]

Coverage remained the same at 95.845% when pulling 8652361276991eabdd97542b23985fae92df2b47 on bitdancer:patch-1 into 992c356b55292b3dc9f6e3bec10b889b7ba36266 on benjamin-hodgson:master.

[benjamin-hodgson]

Thanks for the contribution!