man: Mention Ed448 for CMS with signed attributes is not supported

man: Mention Ed448 for CMS with signed attributes is not supported

Mention that Ed448 keys cannot currently be used for CMS with
signed attributes since RFC 8419 requires id-shake256-len be used,
which is not currently supported by OpenSSL.

Resolves: 30291
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
MergeDate: Tue Mar 17 16:20:20 2026
(Merged from https://github.com/openssl/openssl/pull/30312)

(cherry picked from commit 5041f15320e76de26e5524dcaaba94b1780c99a8)