Since ActionView::Helpers::ActiveRecordHelper is removed.

I've intentionally left out some spots as they're the expected output, and since Hash#inspect yields `=>` I guess it may be confusing for beginners to see one syntax in the guide and a different one in their system.

See 8db41ff7 why

I believe this paragraph should move to the end as it's a little out of the scope of Rails guides.

Using these words without quotes (example sentence: Before filters are run) can be confusing to a newcomer.

I think doing `@comments.name` is a little confusing, just doesn't sounds right.

Updated the preformance_testing guide to reflect the fact that prof-rails no longer needs to be fetched from github