Update uris

I'm not sure this is the best place to document durability guarantees, but either way we need a bit more detail on this topic. For this page, I'd recommend that document the `DatasetWriter` lifecycle:

1. `Dataset#newWriter()` returns a `DatasetWriter`
1. `DatasetWriter#write()` is used to write an entity to a Dataset
1. `DatasetWriter#flush()` will attempt to push buffered from the client to the remote hosts. Some Dataset implementations may not implement this method depending on the guarantees available to the underlying storage system. In particular, when using HDFS-backed datasets the Parquet format does not implement `flush()` by default. 
1. `DatasetWriter#sync()` will execute a `flush()` as well as wait for the remote hosts to force their buffers to stable storage (i.e. hard disks). Like `flush()`, `sync()` depends on support of the underlying format and again, Parquet is not supported by default.
1. `DatsetWriter#close()` will close the writer and if it returns without throwing an exception, then any entity that was successfully written with `DatasetWriter#write()` will be safely stored to stable storage.

We also need somewhere to go into more details on the durability guarantees of different formats and how you can enable it for Parquet with a trade-off in write performance.