During studying Ext documentation of new 3.0RC1 release, I came up with an idea that it could be nice to have new stuff highlighted. MindPatterns has got the similar idea here https://www.sencha.com/forum/showthread.php?43016-Which-Design-Pattern-ExtJS-follows
The result below:
Save the adds.js from adds.zip in ./ext-3.0-rc1/docs/resources/. Then alter ./ext-3.0-rc1/docs/index.html like this. Learn more here about Ext Js WordPress.
It adds a new block on the right on API Home where you can enter a link to other Ext docs. (In my case I wanted to compare docs from 2.2.1 with 3.0RC1.). Then, when opening new tab, two XHRs are made, for files from ./ext-3.0-rc1/docs/output/ and files from ../../ext-2.2.1/docs/output (in my case). After that, the simple comparing takes place. As the result, you are able to see new stuff highlighted, changed stuff with a red underline. Additionally, at the top of the class page new section (Comparison Info) is added. It presents some statistics and details of comparison. Of course, this does not exempt you from the studying the sources, but it highlights places you should check. I have made it for my own needs, but perhaps it can help someone else.