fixes http://eigen.tuxfamily.org/bz/show_bug.cgi?id=568
(files from previous build were kept on the server, with outdated/garbled
information)

The documentation update script now wipes build/doc/html
before rebuilding stuff. Most of the time/cpu consuming is spent in
compiling snippets, so we don't loose that much.
fix a weird bug where a space was missing before a link