worktree: say that "add" takes an arbitrary commit in short-help

worktree: say that "add" takes an arbitrary commit in short-help

c4738aed ("worktree: add can be created from any commit-ish",
2017-11-26) taught "git worktree add" to start a new worktree
with an arbitrary commit-ish checked out, not limited to a tip
of a branch.

"git worktree --help" was updated to describe this, but we forgot to
update "git worktree -h".

Signed-off-by: Junio C Hamano <redacted>