Revert "Revert "update doc, assert_template also accepts false""

Reason: docrails policy is very strict about code. No matter how
innocent the change, if it is not documentation it needs to go
through a PR to rails/rails.

Plase see the warning box at the bottom of

    http://edgeguides.rubyonrails.org/contributing_to_ruby_on_rails.html#contributing-to-the-rails-documentation

This reverts commit e1c5d24369440857de1402d61c85d982b29d8019.