ExtRepository Javadoc

Hey Jim, two notes:

I edited the `package.html` as close to others I could find. I wrapped the text in 80 columns and used similar formatting.

I had a few questions on the `ExtRepository` Javadoc, and was sent some very useful answers to my questions from Ivan. I've forwarded this info to you via email. This extra info was added in my *Clarify ExtRepository Javadoc* commit. 

Thanks!

https://issues.liferay.com/browse/LRDOCS-933