Update to the "linked checkout" in 2.5.0-rc1.

Instead of "checkout --to" that does not do what "checkout"
normally does, move the functionality to "git worktree add".

As this makes the end-user experience of the "worktree add" more or
less complete, I am tempted to say we should cook the other topic
that removes the internal "new-worktree-mode" hack from "checkout"
a bit longer in 'next', and release 2.5 final without that one.

* es/worktree-add:
  Documentation/git: fix stale "MULTIPLE CHECKOUT MODE" reference
  worktree: caution that this is still experimental
  Documentation/git-worktree: fix stale "git checkout --to" references