amount: Improve docs on div by weight

Make an effort to improve the docs on `Amount::checked_div_by_weight`
and `ops::Div<weight> for Amount`.

units/src/amount/unsigned.rs

@@ -235,7 +235,7 @@ impl Amount {
     ///
     /// Be aware that integer division loses the remainder if no exact division
     /// can be made.  This method rounds up ensuring the transaction fee-rate is
-    /// sufficient.  If you wish to round-down, use the unchecked version instead.
+    /// sufficient. If you wish to round down use `amount / weight`.
     ///
     /// [`None`] is returned if an overflow occurred.
     #[cfg(feature = "alloc")]

units/src/fee_rate.rs

@@ -211,6 +211,10 @@ impl Mul<Weight> for FeeRate {
 impl Div<Weight> for Amount {
     type Output = FeeRate;
 
+    /// Truncating integer division.
+    ///
+    /// This is likely the wrong thing for a user dividing an amount by a weight. Consider using
+    /// `checked_div_by_weight` instead.
     fn div(self, rhs: Weight) -> Self::Output { FeeRate(self.to_sat() * 1000 / rhs.to_wu()) }
 }