Default number of buckets does not include system buckets

When using default limit, which is 20, for the number of buckets to be returned, `GET /buckets` endpoint returns 22 buckets, which includes two system buckets `_tasks` and `_monitoring`.
If that is intentional then we would need to update swagger doc to mention that default limit value does not include system buckets.
The same happens when setting limit to any other number less or equals to 100 (currently we won't return more than 100 buckets).
One thing is that it is not happening all the time.

Start by reproducing GET /buckets with the default limit and with limits up to 100, checking whether _tasks and _monitoring are included consistently. Read the endpoint behavior and its Swagger documentation; done means either correcting the returned count or documenting the system-bucket exception, with the intermittent behavior explained.

Written by the indexing model from the issue text.