PR proposal: clarify host header behavior and directly link to docs

**PR proposal:** Provide direct link to docs and httpHostHeader example

**Changes:** [diff in my repository](https://github.com/cloudflare/argo-tunnel-examples/compare/master...ryandellolio:argo-tunnel-examples:clarify-host-header-behavior)

_Filing this as a result of my experience, and opening this issue first per the contributing guidelines._

**Rationale**

The httpHostHeader key defaults to "" per the documentation, which overrides what the browser sends and requires this key to be set for most k8s ingress controllers configured on a per host basis. Most k8s implementations will have cloudflared call a common ingress controller which, if looking for a specific host, as is usually the case, and the request comes in without it, will 404. Therefore, I recommend including the example.

(Even better, in the future, would love to see an option to preserve this header in named tunnels. A workaround is to directly attach cloudflared to the ingress service.)

In addition, based on the difficulty navigating documentation to find these parameters and, in particular, their defaults, I recommend including the direct link to those docs similar to the ingress doc link further down in the config.yaml example.

Start with the linked comparison and inspect the config.yaml example and the documentation it references. Confirm the example explains httpHostHeader and includes a direct link to the relevant parameter documentation; completion should make the default and Kubernetes ingress behavior easier to find.

Written by the indexing model from the issue text.