Give player message informing them they can use #enhance

Triggers when you feel more confident in your skills. This is to address
a problem I have heard about several times from newer players: unless
you pay close attention to the guidebook, nothing in the game actually
indicates that you can level up your abilities and how to do it.

Experienced players don't need this message, of course; they can hide it
via MSGTYPE if they really hate it, but I additionally added a clause
that prevents this message from being displayed more than once per game
session. (It didn't seem important enough to make a save field for.)

src/weapon.c

@@ -69,11 +69,17 @@ static void
 give_may_advance_msg(skill)
 int skill;
 {
+    static boolean got_enhance_tip = FALSE;
     You_feel("more confident in your %sskills.",
              (skill == P_NONE) ? ""
                  : (skill <= P_LAST_WEAPON) ? "weapon "
                      : (skill <= P_LAST_SPELL) ? "spell casting "
                          : "fighting ");
+
+    if (!got_enhance_tip) {
+        pline("(Use the #enhance command to advance them.)");
+    }
+    got_enhance_tip = TRUE;
 }
 
 /* weapon's skill category name for use as generalized description of weapon;