Improve varargs/variadics docs

[rukai]

closes https://github.com/astonbitecode/j4rs/issues/123

This PR does two important things:

• includes the word varargs in the docs, which is the actual term used by java, and the term that I searched for in the readme. When I found no results for varargs in the readme I incorrectly concluded that j4rs did not support varargs.
• Describes in words that varargs is supported by passing in an array. I am not very experienced in java so I did not know this and didnt realize thats what the example given was doing. A written description would have cleared up my confusion a lot sooner, so I've added one.