feature #3936 Varnish only takes into account max-age (gonzalovilaseca)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #3936).

Discussion
----------

Varnish only takes into account max-age

| Q             | A
| ------------- | ---
| Doc fix?      | [no]
| New docs?     | [no] (PR # on symfony/symfony if applicable)
| Applies to    | [Symfony version 2.3]
| Fixed tickets |
Varnish only takes into account max-age, Symfony by default returns Cache-Control: no-cache so Varnish caches it anyway. You need to specify:
```
if (beresp.http.Pragma ~ "no-cache" ||
             beresp.http.Cache-Control ~ "no-cache" ||
             beresp.http.Cache-Control ~ "private") {
    return (hit_for_pass);
}
```
In order to avoid Varnish from caching content.

Commits
-------

466bd6a Update varnish.rst
8848549 Update varnish.rst
f958391 Varnish only takes into account max-age

Author: weaverryan

cookbook/cache/varnish.rst

@@ -57,6 +57,14 @@ Symfony2 adds automatically:
             // For Varnish < 3.0
             // esi;
         }
+        /* By default Varnish ignores Cache-Control: nocache 
+        (https://www.varnish-cache.org/docs/3.0/tutorial/increasing_your_hitrate.html#cache-control),
+        so in order avoid caching it has to be done explicitly */
+        if (beresp.http.Pragma ~ "no-cache" ||
+             beresp.http.Cache-Control ~ "no-cache" ||
+             beresp.http.Cache-Control ~ "private") {
+            return (hit_for_pass);
+        }
     }
 
 .. caution::