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.

deltachat-ffi/deltachat.h

@@ -4588,6 +4588,9 @@ int dc_msg_has_html (dc_msg_t* msg);
   * - @ref DC_DOWNLOAD_IN_PROGRESS    - Download was started with dc_download_full_msg() and is still in progress.
   *                                     If the download fails or succeeds,
   *                                     the event @ref DC_EVENT_MSGS_CHANGED is emitted.
+  *
+  * - @ref DC_DOWNLOAD_UNDECIPHERABLE - The message does not need any futher download action.
+  *                                     It was fully downloaded, but we failed to decrypt it.
   * - @ref DC_DOWNLOAD_FAILURE        - Download error, the user may start over calling dc_download_full_msg() again.
   *
   * @memberof dc_msg_t
@@ -6445,6 +6448,11 @@ void dc_event_unref(dc_event_t* event);
  */
 #define DC_DOWNLOAD_FAILURE        20
 
+/**
+ * Download not needed, see dc_msg_get_download_state() for details.
+ */
+#define DC_DOWNLOAD_UNDECIPHERABLE 30
+
 /**
  * Download in progress, see dc_msg_get_download_state() for details.
  */

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,

node/lib/constants.ts

@@ -28,6 +28,7 @@ export enum C {
   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,