changing bookmark alert to be more accurate to what it actually does

public/language/en_GB/topic.json

@@ -29,7 +29,7 @@
 	"flag": "Flag",
 	"locked": "Locked",
 
-	"bookmark_instructions" : "Click here to return to your last position or close to discard.",
+	"bookmark_instructions" : "Click here to return to the last unread post in this thread.",
 
 	"flag_title": "Flag this post for moderation",
 	"flag_confirm": "Are you sure you want to flag this post?",

public/language/en_US/topic.json

@@ -25,7 +25,7 @@
     "tools": "Tools",
     "flag": "Flag",
     "locked": "Locked",
-    "bookmark_instructions": "Click here to return to your last position or close to discard.",
+    "bookmark_instructions" : "Click here to return to the last unread post in this thread.",
     "flag_title": "Flag this post for moderation",
     "flag_confirm": "Are you sure you want to flag this post?",
     "flag_success": "This post has been flagged for moderation.",