Add reverse-proxy troubleshooting section to Kubernetes docs

[IgorBenko]

Will look like this:

I also mentioned another possible issue if they are using different hostnames on their internal network for the upstream just to be sure.

[stevenrombauts]

I think the last changes make things a little less clear, if you're not using Nginx as reverse proxy. It's a little bit confusing because the entire text now assumes you do, until the last sentence.

What confused me was the mention of proxy_pass in the third paragraph, if we can just make it clear that this is a nginx-specific directive, then everything sounds much more clearer to me. What do you think?

[pscheit]

please send another pr to update the docs reference

[IgorBenko]

@pscheit Thanks for the hint about updating the docs reference, that would have certainly slipped my mind 🙂