We can make use of a JDK version x HBase version matrix to explain which JDK version is supported and required.
0.94, 0.96, and 0.98 releases all support JDK6 and JDK7. For 1.0, there is a discussion thread to decide whether to drop JDK6 support.
There has been some work to support JDK8. We can also document that.