Add callbacks documentation for upgrading to 4.1.

It is now expected in 4.1 to use an explicit block rather than implicit
when setting callbacks through ActiveSupport::Callbacks. This commit
highlights this new expectation as part of the upgrading documentation.

This PR is on request of @chancancode in #16075.

Documentation change only.

Note: I forgot to add [ci skip] to the commit - I will remember for next
time.
I think it's confusing that the newer way is on top. It looks like most sections don't show old way vs new way. Maybe flip them and put "Previously in Rails 4.0" and "Now in Rails 4.1" or something like that.