Document nested attributes as hash of hashes

Document the possibility to use a hash of hashes for nested attributes for a one-to-many association (in addition to the documented possibility to use an array of hashes).

Align indentation in comments.
Merge pull request #9688 from alexeymuranov/doc-nested-attributes-3-2

3-2-stable documentation: the possibility to use a hash of hashes for nested attributes
Fix "Scaling Rails Screencasts" link in caching guide

Closes #10241 [ci skip]
Use secure source in generated Gemfile for creating new plugin. Closes #10245
Merge pull request #10246 from prathamesh-sonpatki/3.2.13-https-rubygems

Using secure source https://rubygems.org when generating new plugin with Rails 3.2
adding a test for root path in the app
just clear the caches on clear! rather than replacing. fixes #10251