Add docs noting requirements for SM2 signing

Reviewed-by: Paul Yang <kaishen.yy@antfin.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #23887)

doc/man7/EVP_PKEY-SM2.pod

@@ -38,6 +38,9 @@ Getter that returns the default digest name.
 B<SM2> signatures can be generated by using the 'DigestSign' series of APIs, for
 instance, EVP_DigestSignInit(), EVP_DigestSignUpdate() and EVP_DigestSignFinal().
 Ditto for the verification process by calling the 'DigestVerify' series of APIs.
+Note that the SM2 algorithm requires the presence of the public key for signatures,
+as such the B<OSSL_PKEY_PARAM_PUB_KEY> option must be set on any key used in signature
+generation.
 
 Before computing an B<SM2> signature, an B<EVP_PKEY_CTX> needs to be created,
 and an B<SM2> ID must be set for it, like this: