Support retrieval of _current snapshot status

Today the [get snapshot status docs](https://www.elastic.co/guide/en/elasticsearch/reference/current/get-snapshot-status-api.html#get-snapshot-status-api-desc) indicate that it's expensive to retrieve the status of completed snapshots and suggests that you can use the string `_current` to retrieve currently-running snapshots. It's true that getting the status of completed snapshots is expensive, but it's not true that you can use `_current` in this way: that only works for the get snapshots API. However it would be awfully useful to support this feature in the snapshot status API too, seeing as how getting the status of currently-running snapshots is the cheapest way to use this API.

(you can use `GET _snapshot/my_repository/_status` to get the status of all running snapshots, but this is undocumented today which needs addressing too)

Start with the linked Get snapshot status API documentation and compare its `_current` behavior with the Get snapshots API. Locate the snapshot status API entry point and its tests, then verify the requested current-snapshot behavior and the documented all-running-snapshots request. Done means the supported requests and documentation agree.

Written by the indexing model from the issue text.