Document the change from a hook to an event for attribute switching Ajax refresh altering

[rszrama]

See this question and my answer / sample code:

https://drupal.stackexchange.com/questions/293877/commerce-hook-attributes-refresh-alter/294092?noredirect=1

[rszrama]

(Was thinking this should be part of the “differences from 1.x to 2.x” docs … both as a specific example but possibly summarized as a general reminder of the switch from hooks to events.)