RFR: 8206318: Enhance package documentation for internal javadoc packages

[Jonathan Gibbons]

Please review a relatively simple noreg-doc update for the javadoc 
internal package-info files.

This is hopefully the beginning of an ongoing campaign to improve the 
internal documentation
for the design and implementation of the javadoc tool.

JBS: https://bugs.openjdk.java.net/browse/JDK-8206318
Webrev: http://cr.openjdk.java.net/~jjg/8206318/webrev.00/

-- Jon