package/cups: store web-interface files under /usr/share/cups/doc-root

The web-interface files (~1.8MB) are by default installed under
/usr/share/doc/cups, which is unfortunate as Buildroot removes usr/share/doc
in target-finalize, breaking the webui.

As a fix, store the web-interface files under /usr/share/cups/doc-root,
similar to how it is done in Debian.

Signed-off-by: Alexey Lukyanchuk <skif@skif-web.ru>
[Peter: use --with-docdir, update description]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>