Update documentation for SSL_set1_sigalgs()

These functions can now take both "sig+hash" strings and
algorithm-specific identifiers like "rsa_pss_pss_sha256" that
indicate a particular entry from the TLS signature algorithm
registry.

Also clarify that only the "_list" form allows for the new-style names
(the non-"list" interfaces take sig and hasn NIDs, which cannot
access all of the new-style schemes).

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5068)