Clarify wsgi_app object purpose in example
Clarify wsgi_app object purpose in example

I've spent a while trying to undestand why sessions example does not work out-of-the-box. After googling "KeyError beaker.session" I've found the discussion https://groups.google.com/forum/#!topic/modwsgi/lyRhHIr48nk where people pointed out to this misleading inaccuracy. We should explain to newcomer that old handler ``simple_app`` is overrided by new one created by Beaker's Middleware.
jvanasco/beaker,masayuko/beaker