Update pkeyutl documentation about the digest option

DSA can accept other digests other than SHA1. EC ignores the digest option
altogether.

Fixes #8425

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8432)

(cherry picked from commit 6a6d9ecd1dff669c162e8ab940dac5db2e82679d)