ZF-3539: Document the test() method in Zend_Cache
Right now there is no API call to read the metadata, besides test(), which does not seem to be documented and I only found out it returns the mtime, while reading the source.
This makes it possible to implement a "fallback" solution. For example one can store an external feed in Zend_Cache. The lifetime is set to lets say 1 day. When trying to update the cache one can first check if the mtime is older than lets say 1 hour. If so an attempt is made to read the data from the original source. If that request fails, I can still fallback on the data in the cache for 1 day since the last request succeeded.