An android project in eclipse don't provide JavaDoc nor sources to android.jar out of the box. Which is quite annoying once you start doing some real development, especially for a first timer who don't know the API too well (like yours truly).
Here is how I fixed it, by checking out the source.
#git clone git://android.git.kernel.org/platform/frameworks/base.git android-sources
#git branch -r
origin/HEAD -> origin/master
I'm using 1.5 aka Cupcake, so I'll checkout that
#git checkout origin/cupcake-release
assuming you've got the sdk in /opt/ like me
#ln -s ~/src/android-sources sources
That's it, restart Eclipse and you should get the source (with javadoc) when stepping into android classes.
Still not getting the source? See the follow up made after having another fight on this.