[WIP] Cookbook grammar and style fixes

This is a follow-up for #4679 

It Fixes some general issues in grammar, style and readability.

(replaces #4693 which was the wrong target branch)

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | >= 2.3
| Fixed tickets | 
yep, i rewrote that section a few days ago. not sure what exactly is unclear. the X-Forwarded-Port needs to be set to whatever port your setup is using to receive external connections. if that would be 1234 then links will be http://example.com:1234/...

the tricky bit is that varnish may be the outmost system, or there could be something else like a load balancer or firewall that re-routes to a different port. most of the time, the external port will be 80. if you are using https, varnish does not support ssl and thus needs a proxy in front of it that terminates ssl, and then the port will almost always be 443.

does that help? happy to help more if i can, but would need a question or a proposal to comment on.