[DOCS] Document _cat/tasks API

[pickypg]

Now that we have _cat/tasks, we should add it to the list of documented _cat APIs as well as describe what the various fields mean.

[debadair]

Documented, but not included in the list of cat APIs in the Reference section.
https://github.com/elastic/elasticsearch/blob/5772c1c7dd343b77af082d37db992b0aad68ae3e/docs/reference/cluster/tasks.asciidoc