Not sure which came first, since the 1996 UK Cluedo edition also included descriptions. However, I think the UK edition was the first to include descriptions of the weapons.
So, were the 1996 & 2000 UK editions the only ones to include descriptions of the weapons?
I think descriptions of the suspects have been standard in all editions since the 50th Anniversary edition. Yet I don't think any other editions have included room and weapons descriptions since the UK 2000 edition, except Super Cluedo in 2006 (which weren't so much descriptions as scene setting).
I really enjoyed reading the descriptions of the rooms and weapons on the UK Cluedo editions, as I did the room descriptions of the FM edition – in fact it's almost hard to believe they skipped the opportunity there to provide detailed descriptions about the weapons ... perhaps they were worn out after the elaborate room descriptions.

If so, it's too bad they dropped these elements from the game.
So, who exactly do we have to thank for the current descriptions we enjoy today? The Franklin Mint, Waddingtons, or (gulp) Hasbro which owned Cluedo lock, stock & barrel at that point?