fix reversion on skipping dead keys in --all/bare

fix reversion on skipping dead keys in --all/bare

Fix a reversion that made dead keys not be skipped when operating on all
keys via --all or in a bare repo. (Introduced in version 8.20200720)

Also, improved the documentation of git-annex-dead, it does not only apply
to fsck --all.

Also, made git-annex fsck, when run on a file whose key is dead, display
that. Before, it displayed that only when run with --all, but with this
fix, it skips dead keys with --all. But it can still be run on a file that
uses a dead key, and displaying "This key is dead" explains to the user
why it does not consider missing content for it to be a problem.

Sponsored-by: k0ld on Patreon