Document how web paths are build

There are common pitfalls that can be easily avoided with some
configuration.
Closes #828

Author: greg0ire

Resources/doc/cache-resolver/web_path.rst

@@ -6,6 +6,17 @@ Web Path Resolver
 
 The ``WebPathResolver`` resolver enabled cache resolution using the
 web path of your application.
+The resulting web path is computed by taking into account the request context,
+notably to decide which scheme to use. If you use a proxy to decrypt incoming
+TLS traffic, and need the resolver to generate https urls, you will need to
+`list trusted proxies in the configuration of the framework`_. If you also
+`embed controllers`_ in your templates, you will need to add the localhost to
+that list.
+The request context is also used to determine the port to specify, should it
+differ from standard HTTP ports like ports 80 or 443.
+
+.. _`list trusted proxies in the configuration of the framework`: http://symfony.com/doc/current/components/http_foundation/trusting_proxies.html
+.. _`embed controllers`: https://symfony.com/doc/current/templating/embedding_controllers.html
 
 
 Configuration