[Minor] Improve content rule descriptions

conf/scores.d/content_group.conf

   }
   "PDF_LONG_TRAILER" {
     weight = 0.2;
-    description = "There is an PDF with a long trailer";
+    description = "There is an PDF with a long trailer in the message";
     one_shot = true;
   }
   "PDF_MANY_OBJECTS" {
     weight = 0;
-    description = "There is a PDF file with too many objects";
+    description = "There is a PDF with too many objects in the message";
     one_shot = true;
   }
   "PDF_TIMEOUT" {
     weight = 0;
-    description = "There is a PDF file that caused timeout in processing";
+    description = "There is a PDF in the message that caused timeout in processing";
     one_shot = true;
   }
 }
-