If you see in your Eclipse on mousover this:

 

Instead of the original Android Javadoc entry:

 

Follow this steps to get it back again:

  1. In Eclipse, right click on your Android project and select Properties
  2. On the menu on the left, select “Java Build Path
  3. On the right side, select the “tab” labelled “Libraries“.
  4. Here you should see the Android SDK that you’re targeting. For example: “Android 2.2”.
  5. Click on the arrow to the left of the Android SDK to expand the sublevels.
  6. Find “Android.jar” and click on the arrow to the left to expand it.
  7. You should see a setting called “Javadoc location“. Select it and click on the “Edit” button.
  8. At the top, select the path to your javadocs.
    This is usually “path_to_android_sdk/android-sdk-windows/docs/reference”.
  9. Click on “validate“. You should be all set now!