We replace the "flush" and "delete" by "delete" and "deletedocuments"
logic. This makes it more obvious what will happen, without reading the
docs.
Also we kept the logic to always provide the index name, as
we will need them in the future. Due to elasticsearch v6 changes no
types are allowed in the same index in the future. Therefore we need to
make it possible to use different indexes in the future, leading to the
need to provide the document type all the time.
* Add some missing documentation.
* Adjust some documentation due to breaking changes.
* Adjust structure of documentation for easier changelog rendering and
browsing.