We have a few leftover mentions of `zen` discovery, mostly for
historical/BwC reasons, which this commit removes.
Prior to this commit the default value for `discovery.type` was `zen`
but this was not written down anywhere or officially supported: the two
options were to set it to `single-node` or to omit it entirely. This
commit changes the default to `multi-node` and documents this.
Co-authored-by: Adam Locke <adam.locke@elastic.co>
8cf4c7b6fb