Repurpose flag to be more general

The flag means that the update shouldn't be broadcast to the network: it
must be kept between the two peers and only included in payment errors.

Author: t-bast

07-routing-gossip.md

@@ -425,13 +425,12 @@ individual bits:
 | 0             | `direction` | Direction this update refers to. |
 | 1             | `disable`   | Disable the channel.             |
 
-The `message_flags` bitfield is used to provide additional details about how to
-interpret some of the message fields:
+The `message_flags` bitfield is used to provide additional details about the message:
 
-| Bit Position  | Name                      | Field                            |
-| ------------- | ------------------------- | -------------------------------- |
-| 0             | `must_be_one`             |                                  |
-| 1             | `scid_alias`              | `short_channel_id`               |
+| Bit Position  | Name           |
+| ------------- | ---------------|
+| 0             | `must_be_one`  |
+| 1             | `dont_forward` |
 
 The `node_id` for the signature verification is taken from the corresponding
 `channel_announcement`: `node_id_1` if the least-significant bit of flags is 0
@@ -446,10 +445,7 @@ The origin node:
   `announce_channel` bit was not set).
     - MUST set the `short_channel_id` to either an `alias` it has
     received from the peer, or the real channel `short_channel_id`.
-    - if it sets `short_channel_id` to an `alias`:
-      - MUST set `scid_alias` to 1 in `message_flags`
-    - otherwise:
-      - MUST set `scid_alias` to 0 in `message_flags`
+    - MUST set `dont_forward` to 1 in `message_flags`
     - MUST NOT forward such a `channel_update` to other peers, for privacy
     reasons.
     - Note: such a `channel_update`, one not preceded by a