There are two possibilities:
(1) you still have a package installed that owns those files. This
is likely the case for the stuff under /usr/share/doc.
You can find out which packages have installed a file with
dpkg -S (filename).
(2) the package is buggy and doesn't clean up after itself. This is
likely the caes for stuff under /etc.
In this case, slap the maintainer with a bug report.
Daniel