FIX Clone Config_LRU incl. objects in array

Caused key confusions when using Config::nest()/unnest()
Merge pull request #1335 from chillu/pulls/config-clone-nest

FIX Clone Config_LRU incl. objects in array
Fixed _config.php references and usage in docs
Config docs improvement: @config and LSB mentions

Also moved some less important "notes" further down the page.
The doc is still written too much like a technical spec, we need
something that's more accessible to beginners.
Fixed Email.admin_email config static naming
Removed unused DataExtension::$extendable_statics

Oversight from cleanup in 876f4c5