Merge branch 'es/wt-add-detach'

"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