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

Add `#environment`

Update function signatures for `#gem` and `#generate`.

Fixes #2468.

It's actually the FileStore.

Fixes #8130.

The shortcut ampersand syntax of `select.map(&:field)` is same thing as `.plunk(:field)`. document that