[SPARK-7890] [DOCS] Document that Spark 2.11 now supports Kafka

Remove caveat about Kafka / JDBC not being supported for Scala 2.11

Author: Sean Owen <sowen@cloudera.com>

Closes #6470 from srowen/SPARK-7890 and squashes the following commits:

4652634 [Sean Owen] One more rewording
7b7f3c8 [Sean Owen] Restore note about JDBC component
126744d [Sean Owen] Remove caveat about Kafka / JDBC not being supported for Scala 2.11

docs/building-spark.md

@@ -130,9 +130,7 @@ To produce a Spark package compiled with Scala 2.11, use the `-Dscala-2.11` prop
     dev/change-version-to-2.11.sh
     mvn -Pyarn -Phadoop-2.4 -Dscala-2.11 -DskipTests clean package
 
-Scala 2.11 support in Spark does not support a few features due to dependencies
-which are themselves not Scala 2.11 ready. Specifically, Spark's external 
-Kafka library and JDBC component are not yet supported in Scala 2.11 builds.
+Spark does not yet support its JDBC component for Scala 2.11.
 
 # Spark Tests in Maven