Expand tls plugin docs regarding SSL cert chains and testing

Perhaps counterintuitively, IO::Socket::SSL will only accept intermediate certificates in the server cert file (SSL_cert_file), not the certificate authority cert file (SSL_ca_file).  Intermediate certs must be given out any time a multi-link cert chain is used, which is somewhat common with CAs, especially in their cheaper/free lower-verification level certificate products.

Also add a mention of testing with openssl's s_client mode, which gives useful feedback on config errors and certificate problems.