RFR: 9/8u40 8062475: Enable hook for custom doc generation

[roger riggs]

To enable generating additional javadoc from the CUSTOM_MAKE_DIR 
directory please review:

Webrev:
http://cr.openjdk.java.net/~rriggs/webrev-custom-javadoc/
Issue:
   8062475: Enable hook for custom doc generation

Thanks, Roger