[SPARK-5800] Streaming Docs. Change linked files according the selected language

Currently, Spark Streaming Programming Guide after updateStateByKey  explanation links to file stateful_network_wordcount.py and note "For the complete Scala code ..." for any language tab selected. This is an incoherence.

I've changed the guide and link its pertinent example file. JavaStatefulNetworkWordCount.java example was not created so I added to the commit.

Author: gasparms <gmunoz@stratio.com>

Closes #4589 from gasparms/feature/streaming-guide and squashes the following commits:

7f37f89 [gasparms] More style changes
ec202b0 [gasparms] Follow spark style guide
f527328 [gasparms] Improve example to look like scala example
4d8785c [gasparms] Remove throw exception
e92e6b8 [gasparms] Fix incoherence
92db405 [gasparms] Fix Streaming Programming Guide. Change files according the selected language