Clarify Caching guide using external materializaion

The caching guide section that relates to external materialization needs to add the following information for clarity:

• the TTL needs to be set at a minimum of 5 minutes (300000) so in cases where the status gets hung at "LOADING" (i.e., the target cache goes down during materialization), the materialization process will recover and restart the loading.

• add a status table "Status" matrix:
  LOADED - means the cache was loaded
  LOADING - means cache is in the process of loading
  FAILED_LOAD - means an error occurred during the load process
  ...

and these should correlate to the section that describes the process in detail.