Update .po and strip down untranslated entries.

This work is done by execute these commands manually:

$ po4a --copyright-holder="The Rust Project Developers" \
    --package-name="Rust" \
    --package-version="0.10-pre" \
    -M UTF-8 -L UTF-8 \
    doc/po4a.conf
$ for f in doc/po/**/*.po; do
>   msgattrib --translated $f -o $f.strip
>   if [ -e $f.strip ]; then
>       mv $f.strip $f
>   else
>       rm $f
>   fi
> done

It should be managed by the build system automatically to use in our
translation workflow, but I've not yet done that.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Add notes for translators.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>