Since Android Development Tools (ADT) r17, projects in eclipse would automatically add any libraries placed in the
libs folder to your build path. This is a great feature but has a pretty horrible side effect in that one cannot attach source code to the libraries in the conventional way under the build path properties page. Fortunately, since the release of ADT r20, there seems to be a work-around which is explained nicely on the following site:
I found that once I had created my x.jar.properties file I had to close and reopen my eclipse project for this to work.