Improve ca_file and cert_file docs

Currently there is no mention of consul supporting intermediate CAs with its TLS configuration, that would be very useful information when you configure consul with your existing PKI system.

Also there is maybe a possible bug, because consul agent currently do not present incomplete certificate chain, but i think technically there is no reason not to (nginx, for example, will happily present broken cert chains) which is not very intuitive when trying out test configs.

Whats more - currently in the docs "certificate" is in singular form, implying that there cannot be more than one certificate in the files