doc(bdk): Clarify the absolute_fee docs

Fixes #1066

crates/bdk/src/wallet/tx_builder.rs

@@ -198,6 +198,10 @@ impl<'a, D, Cs: CoinSelectionAlgorithm, Ctx: TxBuilderContext> TxBuilder<'a, D,
  /// 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