For people who know Apple's SDK...

For people who know Apple’s SDK or like to know the difference between OS versions.

I find this site very useful.
http://codeworkshop.net/objc-diff/sdkdiffs/

Here is the command-line tool.
http://codeworkshop.net/files/objc-diff-0.4.0.zip

And maybe it will be useful to you too.

Regards.