docs: clarify received timestamp

deltachat-ffi/deltachat.h

@@ -3957,14 +3957,16 @@ int64_t          dc_msg_get_timestamp          (const dc_msg_t* msg);
 
 /**
  * Get the message receive time.
- * The receive time is returned as a unix timestamp in seconds.
+ * For webxdc apps, this is the time of the last status update, if any,
+ * also for outgoing messages.
+ * The time is returned as a unix timestamp in seconds.
  *
  * To get the sending time, use dc_msg_get_timestamp().
  *
  * @memberof dc_msg_t
  * @param msg The message object.
- * @return The receiving time of the message.
- *     For outgoing messages, 0 is returned.
+ * @return The receiving time of the message or the time of the last webxdc update.
+ *     0 if there is none, e.g. for most outgoing messages that are not webxdc.
  */
 int64_t          dc_msg_get_received_timestamp (const dc_msg_t* msg);
 

deltachat-jsonrpc/src/api/types/message.rs

@@ -55,7 +55,13 @@ pub struct MessageObject {
 
     timestamp: i64,
     sort_timestamp: i64,
+
+    /// The message receive time.
+    /// For webxdc apps, this is the time of the last status update, if any,
+    /// also for outgoing messages.
+    /// 0 if there is none, e.g. for most outgoing messages that are not webxdc.
     received_timestamp: i64,
+
     has_deviating_timestamp: bool,
 
     // summary - use/create another function if you need it