Add request or index details in Task Management API

[cwurm]

Describe the feature:
It would be nice to see the original request or details about affected indexes In the output of _tasks.

E.g. GET /_tasks?detailed=true&actions=*reindex&pretty provides no information about what it's working on, making it hard to know which task corresponds to which reindexing job currently running.

[clintongormley]

@imotov is this similar to what you're doing in #21740 ?

[imotov]

@clintongormley yes, it's pretty much the same only for a different set of requests.

[imotov]

@cwurm adding a task description is relatively simple process for a single request, however we have close to 100 different requests and it's not really possible (or useful) to add task descriptions to all of them at the same time. I have opened PR for reindex, update-by-query and delete-by-query commands. If you can think of other tasks for which you would like to prioritize adding proper descriptions, please list them on this issue.

[cwurm]

@imotov The one other I can think of is snapshot/restore. However I didn't check if that one already has an informative description.

[imotov]

@cwurm They don't. I just opened #21901 to fix that. I am also planning to add a description to bulk request next. I think that should cover most of the users' needs.