feature #8442 Update http_cache.rst (tabbi89, javiereguiluz)

This PR was merged into the 2.7 branch.

Discussion
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Update http_cache.rst

I want to point out that POST method is cacheable however this is very very narrow case but it is possible. Reading documentation you can learn that POST is not cacheable but this is not true and technically you can cache it.

Commits
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e89aab0 Reworded the explanation abut POST caching
44e569f Update http_cache.rst

Author: wouterj

http_cache.rst

@@ -33,7 +33,6 @@ Nottingham's `Cache Tutorial`_.
 .. index::
    single: Cache; Proxy
    single: Cache; Reverse proxy
-   single: Cache; Gateway
 
 .. _gateway-caches:
 
@@ -286,11 +285,15 @@ Safe Methods: Only caching GET or HEAD requests
 HTTP caching only works for "safe" HTTP methods (like GET and HEAD). This means
 two things:
 
-* Don't try to cache PUT, POST or DELETE requests. It won't work and with good
-  reason. These methods are meant to be used when mutating the state of your application
+* Don't try to cache PUT or DELETE requests. It won't work and with good reason.
+  These methods are meant to be used when mutating the state of your application
   (e.g. deleting a blog post). Caching them would prevent certain requests from hitting
   and mutating your application.
 
+* POST requests are generally considered uncachable, but `they can be cached`_
+  when they include explicit freshness information. However POST caching is not
+  widely implemented, so you should avoid it if possible.
+
 * You should *never* change the state of your application (e.g. update a blog post)
   when responding to a GET or HEAD request. If those requests are cached, future
   requests may not actually hit your server.
@@ -366,3 +369,4 @@ Learn more
 .. _`RFC 7234 - Caching`: https://tools.ietf.org/html/rfc7234
 .. _`RFC 7232 - Conditional Requests`: https://tools.ietf.org/html/rfc7232
 .. _`FOSHttpCacheBundle`: http://foshttpcachebundle.readthedocs.org/
+.. _`they can be cached`: https://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-20#section-2.3.4