feat: Set BccSelf to true when receiving a sync message (#6434)

Fix https://github.com/deltachat/deltachat-core-rust/issues/6433

I at first only wanted to do it any outgoing messages, but @link2xt was
concerned that this may accidentally enable bcc_self, e.g. in the
following case:
- you send out a message
- it's deleted, e.g. via ephemeral messages
- Someone forwards this outgoing message to you again, e.g. via a
mailing list.

src/config.rs

@@ -143,7 +143,10 @@ pub enum Config {
     /// Send BCC copy to self.
     ///
     /// Should be enabled for multidevice setups.
-    /// Default is 0 for chatmail accounts before a backup export, 1 otherwise.
+    /// Default is 0 for chatmail accounts, 1 otherwise.
+    ///
+    /// This is automatically enabled when importing/exporting a backup,
+    /// setting up a second device, or receiving a sync message.
     BccSelf,
 
     /// True if encryption is preferred according to Autocrypt standard.
@@ -384,6 +387,11 @@ pub enum Config {
 
     /// Enable sending and executing (applying) sync messages. Sending requires `BccSelf` to be set
     /// and `Bot` unset.
+    ///
+    /// On real devices, this is usually always enabled and `BccSelf` is the only setting
+    /// that controls whether sync messages are sent.
+    ///
+    /// In tests, this is usually disabled.
     #[strum(props(default = "1"))]
     SyncMsgs,