Review feedback from @gregtatcam:

- In progress
- Add explanatory comments, and start refactoring
  loanComputePaymentParts into functions for readability.

include/xrpl/protocol/STAmount.h

@@ -695,6 +695,17 @@ divRoundStrict(
 std::uint64_t
 getRate(STAmount const& offerOut, STAmount const& offerIn);
 
+/** Round an arbitrary precision Amount to the precision of a reference Amount.
+ *
+ * This is used to ensure that calculations involving IOU amounts do not collect
+ * dust beyond the precision of the reference value.
+ *
+ * @param value The value to be rounded
+ * @param referenceValue A reference value to establish the precision limit of
+ *     `value`. Should be larger than `value`.
+ * @param rounding Optional Number rounding mode
+ *
+ */
 STAmount
 roundToReference(
     STAmount const value,
@@ -702,12 +713,16 @@ roundToReference(
     Number::rounding_mode rounding = Number::getround());
 
 /** Round an arbitrary precision Number to the precision of a given Asset.
+ *
+ * This is used to ensure that calculations do not collect dust beyond the
+ * precision of the reference value for IOUs, or fractional amounts for the
+ * integral types XRP and MPT.
  *
  * @param asset The relevant asset
  * @param value The value to be rounded
  * @param referenceValue Only relevant to IOU assets. A reference value to
  *     establish the precision limit of `value`. Should be larger than
- * `value`.
+ *     `value`.
  * @param rounding Optional Number rounding mode
  */
 template <AssetType A>
@@ -722,6 +737,8 @@ roundToAsset(
     STAmount const ret{asset, value};
     if (ret.asset().native() || !ret.asset().holds<Issue>())
         return ret;
+    // Not that the ctor will round integral types (XRP, MPT) via canonicalize,
+    // so no extra work is needed for those.
     return roundToReference(ret, STAmount{asset, referenceValue});
 }