worktree: clarify --expire applies to missing worktrees

worktree: clarify --expire applies to missing worktrees

The `--expire` option for `git worktree list` and `git worktree prune`
only affects worktrees whose working directory path no longer exists.
The help text did not make this clear, and the documentation
inconsistently used "unused" for prune but "missing" for list.

This updates the help text and documentation to consistently describe
these as "missing worktrees".

Signed-off-by: Sam Bostock <sam@sambostock.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>