Update documentation regarding xml-rpc communication

[dskoda1]

There are a couple of pages of docs about using xml-rpc to communicate with supervisor, however on the main demo page docs/api.rst, there is nothing about specifying the rpcinterface.

So that people in the future don't have to go through the issue I had for a few days of just failing to be able to communicate with the supervisor, I placed the code-block of how to specify the rpcinterface directly into the docs/api.rst page.

[mnaberez]

I think it's fine to provide the reminder, but this identical snippet already exists in the configuration docs. Please update this note to link there rather than repeating it.

[dskoda1]

@mnaberez changes have been made to reference the code snippet in the xmlrpc page.

[mnaberez]

Thanks!