RFR: 8146208: Add a public DocTreeFactory to the Compiler Tree API

[Jonathan Gibbons]

This patch adds a new factory class to the Compiler Tree API for 
creating DocTree nodes.

RFE: https://bugs.openjdk.java.net/browse/JDK-8146208
Webrev: http://cr.openjdk.java.net/~jjg/8146208/webrev.03/

-- Jon