Give a message when throwing ammo without a launcher

This is primarily intended to help new players understand the
mechanics (the "wield the launcher, throw the arrow" sequence may
be unfamiliar to newer players), so the message is worded to
indicate the correct way to use the ammo.

src/dothrow.c

@@ -1607,8 +1607,13 @@ throwit(
                     range = BOLT_LIM;
                 else
                     range++;
-            } else if (obj->oclass != GEM_CLASS)
+            } else if (obj->oclass != GEM_CLASS) {
                 range /= 2;
+                pline("You aren't wielding %s, so you throw your %s by %s.",
+                      an(skill_name(weapon_type(obj))),
+                      weapon_descr(obj),
+                      body_part(HAND));
+            }
         }
 
         if (Is_airlevel(&u.uz) || Levitation) {