Thanks Frugaldude. But just like the suggestion from Retrat1970, that doesn't really limit the search to the DW Forum. It might seems to, depending on the search key, but it doesn't.
For instance, if I specify "Distant Worlds + Troop Generals", it seems, from the first result page, to have returned only results from the DW forum. But the following pages contain results for other games (for instance: "Ultimate General: Civil War"). And it is much more obvious if I use only "Generals" as the search key ("Distant Worlds + Generals). In that case, there are many results for other games, even on the first results page.
But thanks anyway to both you and Retreat1970: your suggestions at least allow to do searches in the forum.