Review/revise X-forwarded reference topic

https://draft-openlibertyio.mybluemix.net/docs/ref/general/#forwarded-header.html

This topic was copied mostly from the old-style doc issue that the developer created. Post-publishing, some urgent fixes have been made but it needs a full review and some revision, inc:

• Move config snippets to generated doc when capability in #206 has been completed.
• Identify and write concept topic.
• Identify and create (if necessary after all generated doc has been checked and enhanced/updated if necessary) reference topic (ie if all the reference info in this topic actually is or could be in the generated docs that relate to this area, we don't need a manually created reference topic any more).
• Remove references to the WebSphere Liberty Knowledge Center. Identify topics that need to be written for OL docs if those links are important and create issues to be prioritised.

Source of info
https://openliberty.io/blog/2018/12/20/http-forwarded-header.html

When complete, redirect the blog post to the concept topic (ask Alasdair how to do the redirect).

Start by reviewing the published X-forwarded reference topic at the linked URL and the Open Liberty blog post, then check the status of capability issue #206. Done means revising or replacing the concept and reference coverage, removing Knowledge Center links, creating follow-up issues where needed, and redirecting the blog post to the concept topic.