Improve user context caching docs (fix #59)

Unset req.http.X-FOSHttpCache-TempCookie

Fix errors and rename HTTP headers

Fix variable name

Fix syntax highlighting

Note that application not Varnish sets Vary header

Fix whitespace and rename HTTP headers

Add note that hash response should be invalidated

Simplify HTTP header names in Varnish config and make them consistent
Merge pull request #66 from FriendsOfSymfony/user-context-docs

Improve user context caching docs