Skip tags in Zend_Cache for Aggregate if not available, better docs

Allows usage of one consistent Zend_Cache backend for all SilverStripe core
storage, greatly simplifying its configuration. This means a call to
Aggregate::flushCache() will indeed flush all caches if the backend doesn't
support tags, including any custom caches defined through SS_Cache. Given
caches are regarded transient that's an acceptable limitation.

We keep having trouble with extended PHPDoc formatting, and have similar
topics covered in Markdown already. Moved docs, added some details about
timeouts and the theory of caching backends.