[SPARK-18138][DOCS] Document that Java 7, Python 2.6, Scala 2.10, Hadoop < 2.6 are deprecated in Spark 2.1.0

## What changes were proposed in this pull request?

Document that Java 7, Python 2.6, Scala 2.10, Hadoop < 2.6 are deprecated in Spark 2.1.0. This does not actually implement any of the change in SPARK-18138, just peppers the documentation with notices about it.

## How was this patch tested?

Doc build

Author: Sean Owen <sowen@cloudera.com>

Closes #15733 from srowen/SPARK-18138.