[development] where to document additions to the core API?

[Moshe Weitzman]

> am i being crazy, and this should just go directly into #64279, after 

is is perfectly reasonable to put new developer functionality into the 
updating page. this is our chance to tell module developers how they can 
improve their modules with our new widgets. you will notice that new 
features are already there such as hook_profile_alter().