Update the format documentation to describe the current backup format.

Update the format documentation to describe the current backup format.

The old documentation referred to the old binary backup format, and was
incomplete at that.  Rewrite it to discuss the current format, including a
discussion of segment/object storage, object references, the format of the
metadata listing, and the root backup descriptor.

The documentation can be improved, and some parts are certainly a bit
spotty, but this gives a good quick overview of the entire format.