docs: explain directory argument to :Bundle

Add some documentation to explain the directory argument (optional second
argument) to the Bundle command.  Maybe fixes #295

Is this already a complete explanation of the argument? Can someone who actually understands the code in autoload/vundle/config.vim please comment on this. The code is not documented and I was not able to figure out if what I describe here is really all that can be achieved.

I also fixed two typos which I found on the way.