Document `detailed` parameter for task manager

[eskibars]

Elasticsearch version: 5.1.1

Describe the feature: Several people have asked how to use the detailed task descriptions + search cancellation. I realized they're probably asking because we haven't updated our task management docs to list the detailed parameter

[nik9000]

I'll put this on my list of things to do but it might be a day or two. If someone else gets to it before I do please unassign me.