internals: document bundle2 format It seems like a good idea to have thorough documentation of the bundle2 data format, including the format of each part and the capabilities. The added documentation is far from complete. For example, we don't fully capture the semantics of each capability and part. But a start is better than nothing, which was pretty much where we were before. Differential Revision: https://phab.mercurial-scm.org/D2298