"git worktree add" learns that the "-d" is a synonym to "--detach"
option to create a new worktree without being on a branch.

* es/wt-add-detach:
  git-worktree.txt: discuss branch-based vs. throwaway worktrees
  worktree: teach `add` to recognize -d as shorthand for --detach
  git-checkout.txt: document -d short option for --detach