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.

doc/fixes3-7-0.txt

@@ -1564,6 +1564,7 @@ scroll of enchant armor formula has changed (in particular, magical armor now
         gains less enchantment when enchanted than nonmagical armor and
         uncursed scrolls can sometimes enchant by more than one point)
 dragonhide can rot
+throwing ammo without a launcher produces a message
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository

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) {