Clarify return values for EVP_DigestVerifyFinal.

[aeijdenberg]

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

[benlaurie]

+1

[richsalz]

How about this instead?

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 to indicate
a more serious error (and sometimes also indicate an invalid signature form).

[aeijdenberg]

Changed as suggested by Rich (with one minor change, removed extraneous "to").

[levitte]

+1

[richsalz]

done in master and 1.0.2; thanks.

[levitte]

Any reason why not 1.0.1?

[richsalz]

oops. and 1.0.1

[levitte]

Thanks :-)