Fixed "divided by zero" error when returning 0 in AMMOCOST hook

* with CheckWeaponAmmoCost=1

Added a value check to the return value of AMMOCOST hook.
Updated hookscripts.md to reflect this change.

artifacts/scripting/hooks.yml

@@ -296,9 +296,11 @@
   id: HOOK_AMMOCOST
   doc: |
     Runs when calculating ammo cost for a weapon. Doesn't affect damage, only how much ammo is spent.<br>
-    By default, weapons can make attacks when at least 1 ammo is left, regardless of ammo cost calculations.<br>
+    By default, a weapon can perform an attack with at least one ammo, regardless of ammo cost calculation.<br>
     To add proper check for ammo before attacking and proper calculation of the number of burst rounds (hook type 1 and 2 in `arg3`), set **CheckWeaponAmmoCost=1** in **Misc** section of ddraw.ini.
 
+    __NOTE:__ The return value must be greater than or equal to 0 to be valid.
+
     ```
     Item    arg0 - The weapon
     int     arg1 - Number of bullets in burst or 1 for single shots