RFR: 8175831 : Provide a javadoc description for jdk.accessibility module

[Philip Race]

Bug: https://bugs.openjdk.java.net/browse/JDK-8175831
webrev: http://cr.openjdk.java.net/~prr/8175831/
Adds missing javadoc description.
I observed that javadoc is not actually generated for this module
and makefiles needed to be updated and made such changes to test it
but the javadoc folks tell me that I should not push those as there are 
changes afoot
that will automate this without needing to add per-module make targets.

-phil.