+ * Builds an unsigned HTLC-Success or HTLC-Timeout transaction from the given channel and HTLC
+ * parameters. This is used by [`TrustedCommitmentTransaction::get_htlc_sigs`] to fetch the
+ * transaction which needs signing, and can be used to construct an HTLC transaction which is
+ * broadcastable given a counterparty HTLC signature.
+ *
+ * Panics if htlc.transaction_output_index.is_none() (as such HTLCs do not appear in the
+ * commitment transaction).