Update view api

Updates based on comments in https://github.com/kite-sdk/kite-docs/pull/63 .
Again, we can remove "implementation" and use just `DatasetWriter`.

I'm not sure if this should be here or in the Restricted Views doc, but the point is to ensure that a writer isn't doing anything unexpected. That is, creating data files in partitions that it isn't "responsible for" in terms of its constraints.

The restricted views link should be `{{site.baseurl}}/Restricted-Views.html`.