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.
[ci skip] Improve callback code example for 4.1 upgrade docs.

The previous code example for the new explicit block requirement when
setting callbacks was a little confusing. This commit makes the example
more obvious.
Merge pull request #16086 from nickpellant/master

Add callbacks documentation for upgrading to 4.1.