bitcoin/signtx: restore comment lost in Rust conversion
A comment was not copied over from C from this commit, when porting
the locktime/rbf checks to Rust:
The dialog is not shown if locktime is 0, as a beginner user should
not be confrontend with RBF, which is enabled by default in the
BitBoxApp.
@@ -616,6 +616,9 @@ async fn _process(request: &pb::BtcSignInitRequest) -> Result<Response, Error> {
// Verify locktime/rbf.
// A locktime of 0 will also not be verified, as it's certainly in the past and can't do any
// harm.
+ //
+ // This is not a security feature, the extra locktime/RBF user confirmation is skipped if the tx
+ // is not rbf or has a locktime of 0.
if request.locktime > 0 && locktime_applies {
// The RBF nsequence bytes are often set in conjunction with a locktime,
// so verify both simultaneously.
