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 a 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