Reflect special `DEFAULT` behavior in ciphers(1)

Actual behavior of DEFAULT is different than currently described.
Rather than actinf as cipher string, DEFAULT cannot be combined using
logical operators, etc.

Fixes #5420.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5428)