[autodoc] Some toplevel documentatioon for Cache.

lib/modules/Cache.pmod/module.pmod

+//! Common Caching implementation
+//!
+//! This module serves as a front-end to different kinds of caching
+//! systems. It uses two helper objects to actually store data, and to
+//! determine expiration policies.
+//!
+//! To create a new cache, do @[Cache.cache]( @[Cache.Storage.Base] storage_type, @[Cache.Policy.Base] expiration_policy )
+//!
+//!
+//! The cache store instances of @[Cache.Data].