doc(bdk): Clarify the absolute_fee docs

Fixes bitcoindevkit#1066

src/wallet/tx_builder.rs

@@ -189,6 +189,14 @@ impl<'a, D: BatchDatabase, Cs: CoinSelectionAlgorithm<D>, Ctx: TxBuilderContext>
     }
 
     /// Set an absolute fee
+    /// The fee_absolute method refers to the absolute transaction fee in satoshis (sats).
+    /// If anyone sets both the fee_absolute method and the fee_rate method,
+    /// the FeePolicy enum will be set by whichever method was called last,
+    /// as the FeeRate and FeeAmount are mutually exclusive.
+    ///
+    /// Note that this is really a minimum absolute fee -- it's possible to
+    /// overshoot it slightly since adding a change output to drain the remaining
+    /// excess might not be viable.
     pub fn fee_absolute(&mut self, fee_amount: u64) -> &mut Self {
         self.params.fee_policy = Some(FeePolicy::FeeAmount(fee_amount));
         self