dm crypt: document encrypted keyring key option

Commit 27f5411a ("dm crypt: support using encrypted keys")
introduced support for encrypted keyring type.

Fix documentation in admin guide to mention this type.

Fixes: 27f5411a ("dm crypt: support using encrypted keys")
Signed-off-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>