Clarify return values for EVP_DigestVerifyFinal.

Previous language was unclear.  New language isn't pretty but I believe
it is more accurate.

Signed-off-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Ben Laurie <ben@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(cherry picked from commit 8cbb048c)