why it is need ..?
If you not have or limited bandwidth internet connecion in your machine at that time it is very helpfull to solve your java programming problems. The java documentation of your project is help to easily understand the classes and functions used in that project.
How to create a java documentation ..?
It is very easy using the javadoc tool you can create a documentation for any java program. First create a documentation for java
In windows
Extract that C:\Program Files\Java\jdk1.6.0_10\src.zip to src folder then
cd src
dir /s /b *.java > files.txt
javadoc -J-Xmx756m @files.txt
In Linux
cd /usr/local/java/jdk1.6.10
find -r *.java > files.txt
javadoc -J-Xmx756m @files.txt