I know this thread is old but I see that sendtoreader.com links to it as a recommended way for deleting documents in bulk. Copying a piece of code one doesn't understand seems a bit risky to me, so I wrote a bookmarklet that does a similar job and its associated web page provides support. Plus you can install the bookmarklet only by dragging it to your bookmarks.
What it does is that it adds checkboxes to each document in your Kindle library. You tick off those that should be discarded and then make them fly with a single click of a button.
Check it out:
Check & Delete.