AS guide: removes the placeholder for extensions to Benchmark because benchmark.rb just reimplements an existing method

AS guide: removes the placeholder for extensions to Exception because exception.rb seems to provide stuff for internal use

AS guide: removes CGI placeholder section,  CGI.escape_skipping_slashes seems something for internal use

AS guide: reflects the guide finally covers only core extensions (is gonna be one of the biggest ones just with that)

Caching guide: documents that page/action caching run in an after_filter, and this spurious cache entries are avoided just by halting the request

Add a :limit option to specify the maximum number of records that can be processed by accepts_nested_attributes_for

Store entire options hash in the class var rather than just the reject_if proc for the nested attributes