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: NRich Salz <rsalz@openssl.org>
Reviewed-by: NBen Laurie <ben@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>

doc/crypto/EVP_DigestVerifyInit.pod

@@ -36,10 +36,11 @@ B<sig> of length B<siglen>.
 EVP_DigestVerifyInit() and EVP_DigestVerifyUpdate() return 1 for success and 0
 for failure.
 
-Unlike other functions the return value 0 from EVP_DigestVerifyFinal() only
-indicates that the signature did not verify successfully (that is tbs did
-not match the original data or the signature was of invalid form) it is not an
-indication of a more serious error.
+EVP_DigestVerifyFinal() returns 1 for success; any other value indicates
+failure.  A return value of zero indicates that the signature did not verify
+successfully (that is, tbs did not match the original data or the signature had
+an invalid form), while other values indicate a more serious error (and
+sometimes also indicate an invalid signature form).
 
 The error codes can be obtained from L<ERR_get_error(3)>.