Clean up documentation about Alt-Svc HTTP header (#4159)

* Clean up documentation about Alt-Svc HTTP header

Some content had been added that described the Alt-Svc header as having
the purpose of “separating the identity and location of a resource”.
But that’s a misleading description — because what the Alt-Svc header is
really about is, allowing a server to indicate that a particular
resource should be loaded from a different server — while still
appearing to the user as if were loaded from the same server.

See https://www.mnot.net/blog/2016/03/09/alt-svc for a great overview.

So this change:

* Replaces the summary in the Alt-Svc article itself, while adding a See
  Also link to that https://www.mnot.net/blog/2016/03/09/alt-svc article.

* Rewrites the description of Alt-Svc in the “Basics of HTTP” article.

* Removes mention of Alt-Svc from the “Identifying resources on the Web”
  article. That article is entirely about URLs and their constituent
  parts; e.g., their path parts. Alt-Svc is not about URLs — it is about
  origins (host/protocol/port) and has nothing do with, e.g., path
  parts. So Alt-Svc doesn’t belong there; it’s a non sequitur there.

* Removes mention of Alt-Svc from the “Resources and URIs” article, for
  reasons similar to the reasons for removing it from the “Identifying
  resources on the Web” article.

Fixes #4125

* Drop Alt-Svc link form “Basics of HTTP” doc

files/en-us/web/http/basics_of_http/identifying_resources_on_the_web/index.html

@@ -20,8 +20,6 @@
 
 <p class="summary">The target of an HTTP request is called a "resource", whose nature isn't defined further; it can be a document, a photo, or anything else. Each resource is identified by a Uniform Resource Identifier ({{Glossary("URI")}}) used throughout HTTP for identifying resources.</p>
 
-<p>The identity and the location of resources on the Web are mostly given by a single URL (Uniform Resource Locator, a kind of URI). There are sometimes reasons identity and location are not given by the same URI: HTTP uses a specific HTTP header, {{HTTPHeader("Alt-Svc")}} when the resource requested wants the client to access it at another location.</p>
-
 <h2 id="URLs_and_URNs">URLs and URNs</h2>
 
 <h3 id="URLs">URLs</h3>

files/en-us/web/http/basics_of_http/index.html

@@ -25,8 +25,6 @@ <h2 id="Articles">Articles</h2>
  <dd>A specific kind of URI that directly embeds the resource it represents. Data URIs are very convenient, but have some caveats.</dd>
  <dt><a href="/en-US/docs/Web/HTTP/Basics_of_HTTP/Resource_URLs">Resource URLs</a> {{Non-standard_Inline}}</dt>
  <dd>Resource URLs, those prefixed with the <code>resource</code> scheme are used by Firefox and Firefox browser extensions to load resources internally, but is also available to some sites the browser connects to as well.</dd>
- <dt>Separating identity and location of a resource: The Alt-Svc HTTP header</dt>
- <dd>Most of the time the identity and location of a web resource are shared, this can be changed with the {{HTTPHeader("Alt-Svc")}} header.</dd>
  <dt><a href="/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types">MIME types</a></dt>
  <dd>Since HTTP/1.0, different types of content can be transmitted. This article explains how this is accomplished using the {{HTTPHeader("Content-Type")}} header and the MIME standard.</dd>
  <dt><a href="/en-US/docs/Web/HTTP/Basics_of_HTTP/Choosing_between_www_and_non-www_URLs">Choosing between www and non-www URLs</a></dt>

files/en-us/web/http/headers/alt-svc/index.html

@@ -9,9 +9,7 @@
 ---
 <div>{{HTTPSidebar}}</div>
 
-<p>The <strong><code style="white-space: nowrap;">Alt-Svc</code></strong> HTTP response
-  header is used to advertise alternative services through which the same resource can be
-  reached. An alternative service is defined by a protocol/host/port combination.</p>
+<p>The {{HTTPHeader("Alt-Svc")}} HTTP header allows a server to indicate that a particular resource should be loaded from a different server — while still appearing to the user as if were loaded from the same server.</p>
 
 <h2 id="Syntax">Syntax</h2>
 
@@ -84,7 +82,5 @@ <h2 id="Browser_compatibility">Browser compatibility</h2>
 <h2 id="See_also">See also</h2>
 
 <ul>
-  <li><a
-      href="/en-US/docs/Web/HTTP/Basics_of_HTTP/Identifying_resources_on_the_Web">Identifying
-      resources on the Web</a></li>
+  <li><a href="https://www.mnot.net/blog/2016/03/09/alt-svc"> Alternative Services</a> (article about <code>Alt-Svc</code> by HTTP Working Group chair Mark Nottingham)</li>
 </ul>

files/en-us/web/http/resources_and_uris/index.html

@@ -27,6 +27,4 @@
  <dd>MIME media types define what kind of document a specific resource is. This article presents both the syntax and the most useful MIME types for use on the Web.</dd>
  <dt><a href="/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types/Common_types">Common MIME types</a></dt>
  <dd>List of common MIME types useful for Web developers.</dd>
- <dt><a href="/en-US/docs/Web/HTTP/Basics_of_HTTP/Separating_identity_and_location_of_a_resource">Separating identity and location of a resource: the Alt-Svc header</a></dt>
- <dd>Even if identity and location are both described using a {{Glossary("URL")}}, they are two different concepts and it is useful sometimes to distinguished between them. This article introduces the {{HTTPHeader("Alt-Svc")}} header.</dd>
 </dl>