Elasticsearch docs update

[lasteris]

Added code snippet about Bulk API usage via low level RestClient.

I've thought, that this is the important thing, which needs to be described in docs, while It's not so obvious how to use (official docs demonstrate only Java API Bulk API usage). User should know how to do it, till this is really convenient way to index data with the best performance.

In code snippet, you can see ByteArrayEntity. I've tried InputStreamEntity instead, but got Stream already closed error.