doc: Explain verify_result_t.status == 0 more

* doc/gpgme.texi (gpgme_verify_result_t): Explain
GPGME_STATUS_NO_ERROR more clearly.

This might help to avoid misunderstandings how the
status can be interpreted and explains why a verify
of unsigned PGP Data returns no error.

As a reaction to CVE-2020-10759 discovered by Justin Steven.