Posters now say that you can trap them (#72974)

## About The Pull Request

Examining a poster will now say that you can trap it with a glass shard

## Why It's Good For The Game

While trapping posters with shards is a fun mechanic, not everyone knows
that you can do that. This PR makes easier to find out about this
mechanic for new players.

## Changelog
🆑
qol: Posters now tell that you can trap them with a glass shard when
examined
/🆑

---------

Co-authored-by: necromanceranne <40847847+necromanceranne@users.noreply.github.com>

code/game/objects/effects/poster.dm

@@ -20,6 +20,10 @@
 	var/poster_type
 	var/obj/structure/sign/poster/poster_structure
 
+/obj/item/poster/examine(mob/user)
+	. = ..()
+	. += span_notice("You can booby-trap the poster by using a glass shard on it before you put it up.")
+
 /obj/item/poster/Initialize(mapload, obj/structure/sign/poster/new_poster_structure)
 	. = ..()