At the Ext core download page it says the the Ext Core 3.1.0 SDK contains "HTML documentation". It also says that to access the docs you should "Unzip the archive to your web server and navigate to http://your-server/<ext-directory>/docs to view the documentation..."
But it looks like with the upgrade from Ext Core 3 to 3.1 the html docs are no longer distributed. Instead there is a file called DOCS.txt which directs you to the Ext web site.
While I agree that the web site is the best place to go for the most up-to-date documentation, sometimes it is necessary to refer to old documentation, such as when maintaining a site where an earlier version has been deployed and the cost of upgrade (e.g. regression testing) is not justified.
I mainly work with the full version of Ext, for which I have a commercial license, but I do recommend Ext Core to my web designer colleagues and I sometimes use it myself. The full Ext comes with a docs folder which allows me to keep the older docs for future reference. Can we please have the same for Ext Core? Failing that, I suggest you amend your downloads page to indicate that it does not contain the docs.