I had an issues with some joomla api. The issue was that I was using the latest version of joomla and couldn't find where a joomla class, that was being called in my code, was derived . I searched the joomla api docs and found nothing relating to the class I was looking for.
I then stumbled across the refactoring change log for joomla and noticed that the function i was after had actually been moved from /libraries/joomla/form to libraries/cms/form.
Why would this be the case? There doesn't seem to be any reference to /libraries/cms in the api docs. How are we suppose to know that there are classes living there? Very confusing.
Since Joomla! 1.6 the code has been progressively split into the Joomla! CMS and the Joomla! Platform. You can see this separation in the github repository - a good place to also keep track of changes that are committed to both the CMS and the Platform. The simplest way is fork each repository and keep track of them.
As classes are updated they may be migrated to the /libraries/cms directory if they apply specifically to the CMS application (e.g. html forms are an application level function not a platform level).
I find the best place to hear about changes under discussion are in the respective Google Groups - Joomla! Platform Development and Joomla! CMS Development
[Edit]
Documentation of class's is the hardest thing to find for Joomla! CMS - there doesn't appear to be a current api listing for it, unlike the Platform API reference. Often it is just easier to read through the code.
It's also worth keeping a watch on the developer sub-site.