Improve help text for project vocabulary, and also display it at admin/content/taxonomy.

One of the help texts trying to be fixed over at http://drupal.org/node/155727 is for the project vocabulary, currently only printed on the project settings page. I fixed the t() stuff to correspond to the recommended way of doing this (http://lists.drupal.org/pipermail/translations/2007-July/000400.html)

While I was at it, I moved the help text into a helper function so that it could be shared by both the admin/project/project-settings page *and* the admin/content/taxonomy/[vid] page if you're on the project type vocabulary. This seems like a nice usability enhancement, to print out help text right there when adding terms to the vocabulary. And, it helps explain why this vocabulary was created in the first place, for folks who find it by browsing admin/content/taxonomy.

Any objections?