[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