Docs for new Client Cert inputs

[nhayfield]

Nathan put in some placeholder help text here but modifications might be needed.
Then new inputs are on the Add Connection page under Advanced Settings.
There is a PR in to add the code but it is waiting on a release of the CLI so that it will work.

#94

[alexfornuto]

@wasaga please confirm: This setting is required for TCP routes that have tls_downstream_client_ca_file, or if client_ca_file is set globally, yes?