Clearer URL and message for clearing CiviCRM cache

civicrm/clearcache can now be used to clear all CiviCRM caches.
civicrm/menu/rebuild is still an 'alias' of civicrm/clearcache

The message that is displayed after calling each of these has been updated to better reflect what actually happens, i.e. Cleared all CiviCRM caches (database, menu, templates).

(still a lot more we could still do a lot more to clean this up) e.g. clearing the session cache, etc. Worth while consolidating that at some point IMO)