String freeze: Add drag-and-drop help to vocabulary listing.

Unfortunate, but true, the vocabulary listing at admin/content/taxonomy is drag-and-drop enabled but the help text does not reflect this. (While I thought we had gotten the last of these with a post-string-freeze patch to the help text for rearranging input format filters, I was wrong, and this one remains undocumented.)

The attached patch adds drag-and-drop instructions (and some good general instruction on vocabularies for good measure) to a new string shown as a new paragraph of help text. Previous strings are unchanged, but translators will have a new paragraph to translate on short notice and at a late date if this patch is committed. A small change to (ever-growing) list of drag-and-drop references in CHANGELOG.txt is also included.

Unfortunately, the behavior on this page is very subtle and is probably worth documenting; the order of the vocabularies in the listing control the order the vocabularies are shown on node add and edit screens. Without a bit of documentation, I don't know a good way that someone would pick that up without some experimentation. (There is no weight column which would otherwise imply that there was an order involved here.)