doc: git-rebase: update discussion of internals

doc: git-rebase: update discussion of internals

- make it clearer that we're talking about a multistep process
- give a more technically accurate description how rebase works with the
  merge backend.
- delete a duplicate explanation of how git rebase skips commits with
  the same textual changes (it's explained in more detail a few lines
  further down)
- remove the explanation of how exactly `--fork-point` and `--root`
  work since that information is in the OPTIONS section
- put all discussion of `ORIG_HEAD` inside the note

Signed-off-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>