Enhance the documentation for the fee_rate method.

crates/bdk/src/wallet/tx_builder.rs

@@ -184,8 +184,13 @@ impl<'a, D, Cs: Clone, Ctx> Clone for TxBuilder<'a, D, Cs, Ctx> {
 // methods supported by both contexts, for any CoinSelectionAlgorithm
 impl<'a, D, Cs: CoinSelectionAlgorithm, Ctx: TxBuilderContext> TxBuilder<'a, D, Cs, Ctx> {
     /// Set a custom fee rate
-    /// The fee_rate method sets the fee rate using a FeeRate type in either satoshis/vbyte (sats/vbyte)
-    /// using FeeRate::from_sat_per_vb(), or satoshis/kwu (sats/kwu) using FeeRate::from_sat_per_kwu().
+    /// The fee_rate method sets the mining fee paid by the transaction as a rate on its size.
+    /// This means that the total fee paid is equal to this rate * size of the transaction in virtual Bytes (vB) or Weigth Unit (wu).
+    /// This rate is internally expressed in satoshis-per-virtual-bytes (sats/vB) using FeeRate::from_sat_per_vb, but can also be set by:
+    /// * sats/kvB (1000 sats/kvB == 1 sats/vB) using FeeRate::from_sat_per_kvb
+    /// * btc/kvB (0.00001000 btc/kvB == 1 sats/vB) using FeeRate::from_btc_per_kvb
+    /// * sats/kwu (250 sats/kwu == 1 sats/vB) using FeeRate::from_sat_per_kwu
+    /// Default is 1 sat/vB (see min_relay_fee)
     pub fn fee_rate(&mut self, fee_rate: FeeRate) -> &mut Self {
         self.params.fee_policy = Some(FeePolicy::FeeRate(fee_rate));
         self