Corrected documentation

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6723 8aca7d54-2c30-db11-9de9-000461428c89
FIX: No new buying obsolete stuff, just allow appending

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6724 8aca7d54-2c30-db11-9de9-000461428c89