Update HMAC_INPUT docs to accurately describe legacy reservations

We recently switched to `ReceiveAuthKey`-based blinded path
authentication, removing various fields used to authenticate
blinded paths from contexts. In doing so we removed
no-longer-needed `HMAC_INPUT`s in offer metadata validation, and
left a comment noting that previously used values should not be
reused.

That comment was slightly incorrect as it indicated some kind of
"backward compatibility" concern, but of course we broke backwards
compatibility when we stopped accepting the previous authentication
scheme.

Instead, here, we update the comment to note that what we're
protecting against is a type confusion attack.

Author: TheBlueMatt

lightning/src/offers/signer.rs

@@ -42,7 +42,7 @@ const WITH_ENCRYPTED_PAYMENT_ID_HMAC_INPUT: &[u8; 16] = &[4; 16];
 // `OffersContext`, but were removed in LDK v0.2 with the introduction of `ReceiveAuthKey`-based
 // authentication.
 // Their corresponding values (`[5; 16]` and `[7; 16]`) are now reserved and must not
-// be reused to preserve backward compatibility.
+// be reused to ensure type confusion attacks are impossible.
 //
 // Reserved HMAC_INPUT values — do not reuse:
 //