Added some javadoc to com.mongodb.DB class

Hi,
I've added some javadoc to DB class.
I hope that could be useful.

That was my first meet of Mongo DB,  I had no experience of working with that DB, so my first look was static code analysis and launching some demo code from QuickTour and other examples.

Please revise, I made all things right.
If ok, I'll continue with DBCollection class

Best regards, Dzmitry.