docs: document DC_DOWNLOAD_UNDECIPHERABLE

This was introduced in https://github.com/deltachat/deltachat-core-rust/pull/4685
for internal use when looking up a chat based on In-Reply-To,
but actually affects the UIs as they should not display Download button
when the message is downloaded but cannot be decrypted.

node/constants.js

@@ -28,6 +28,7 @@ module.exports = {
   DC_DOWNLOAD_DONE: 0,
   DC_DOWNLOAD_FAILURE: 20,
   DC_DOWNLOAD_IN_PROGRESS: 1000,
+  DC_DOWNLOAD_UNDECIPHERABLE: 30,
   DC_EVENT_CHAT_EPHEMERAL_TIMER_MODIFIED: 2021,
   DC_EVENT_CHAT_MODIFIED: 2020,
   DC_EVENT_CONFIGURE_PROGRESS: 2041,