Let equip_unequip work on weapons (xbon)

And fix the options_guide description for it being inaccurate.

crawl-ref/docs/options_guide.txt

@@ -970,9 +970,9 @@ easy_unequip = true
         Allows auto removal of armour and jewellery when dropping it.
 
 equip_unequip = false
-        If this is true, 'W'ear will also allow you to 'T'ake off worn
-        armour, and vice versa, and the same is true for 'P'utting on/
-        'R'emoving jewellery.
+        If this is true, selecting an already-equipped piece of equipment via
+        the 'w', 'W' or 'P' menus (for weapons, armour or jewellery
+        respectively) will unequip that item.
 
 jewellery_prompt = false
         If this is true, equipping rings will always prompt for the slot to

crawl-ref/source/item-use.cc

@@ -487,8 +487,13 @@ bool wield_weapon(bool auto_wield, int slot, bool show_weff_messages,
         return false;
     else if (item_slot == you.equip[EQ_WEAPON])
     {
-        mpr("You are already wielding that!");
-        return true;
+        if (Options.equip_unequip)
+            item_slot = SLOT_BARE_HANDS;
+        else
+        {
+            mpr("You are already wielding that!");
+            return true;
+        }
     }
 
     // Reset the warning counter.