add item.caption(...) function & move item library docs to libitem.md

src/logic/scripting/lua/libs/libitem.cpp

@@ -47,6 +47,13 @@ static int l_item_get_icon(lua::State* L) {
     return 0;
 }
 
+static int l_item_caption(lua::State* L) {
+    if (auto def = get_item_def(L, 1)) {
+        return lua::pushstring(L, def->caption);
+    }
+    return 0;
+}
+
 const luaL_Reg itemlib[] = {
     {"index", lua::wrap<l_item_index>},
     {"name", lua::wrap<l_item_name>},