clarify the documentation on how to configure a cache implementation

clarify the documentation on how to configure a cache implementation
use the popular cache::memcached::libmemcached as an example
the old code was throwing away the first error when creating a cache instance
(it did so using a list and a hashref as args); if the first failed, the 2nd
call would overwrite the first error. This is now fixed
the calling code wrapped the above code in an eval, and then silently discarded
the return value. This is now no longer the case, as the assumption of the code
was to have a working fallback, which is not currently implemented. This resulted
in a FIXME error on the first request to a controller using a cache->get or cache->set