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.