SSL_CTX_set_cert_verify_callback.pod: various corrections and clarifications

SSL_CTX_set_cert_verify_callback.pod: various corrections and clarifications

- Make clear the callback is called whenever a peer certificate has been received,
  which is independent of the verification mode.
- Make clear that a return value > 1 always leads to handshake failure.
- Make clear that in server mode also return values <= 0 lead to handshake failure.
- For client mode replace the incorrect formulation "if B<SSL_VERIFY_PEER> is set"
  by what is actually implemented: "if the verification mode is not B<SSL_VERIFY_NONE>".
- Refer to X509_STORE_CTX_set_error() rather than to internal error variable.

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13937)