doc: fix support bigint64/biguint64 guards

PR-URL: nodejs/node-addon-api#705
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Nicola Del Gobbo <nicoladelgobbo@gmail.com>

napi-inl.h

@@ -1628,6 +1628,10 @@ inline uint8_t TypedArray::ElementSize() const {
     case napi_float32_array:
       return 4;
     case napi_float64_array:
+#if (NAPI_VERSION > 5)
+    case napi_bigint64_array:
+    case napi_biguint64_array:
+#endif  // (NAPI_VERSION > 5)
       return 8;
     default:
       return 0;