docs: gbp-clone: Clarify commands and include --add-upstream-vcs in example

Ready to be merged. See commit messages for details.

Before I finalize this, can you Guido clarify if you prefer gbp clone vcs-git:x or gbp clone vcsgit:x? We can have two supported commands, but the docs is confusing if you give two equal options to do the same thing. Decide which one it is, and then doc focus on that.

Also I was wondering if you want to have -add-upstream-vcs that created the remote upstreamvcs, or if you want to have --add-upstreamvcs or if we could just have that option by default? Users don't need to fetch or push upstream. Just having one extra remote there would not hurt, right?

I realize I am mixing discussions about docs with discussions about features, but by trying to make the docs crystal clear about how to use git-buildpackage, it doubles as a thorough review of the features themselves.