docs: Correct review-noted inaccuracies in cross-scale fix comments

Address review comments on PR #8002:

- LoanPay.cpp: pre-amendment description incorrectly claimed the vault
  pseudo-account received the raw Number. It received the vaultScale-
  rounded value via accountSendMulti; the real asymmetry is vault-side
  between sfAssetsAvailable (rounded) and sfAssetsTotal (unrounded via
  assetsTotalDelta).
- LoanManage.cpp: replace the associateAsset-based justification for the
  T - A >= totalDefaultAmount guard with the actual invariant: the
  fields loanVaultExposure differences are whole multiples of the same
  immutable 10^sfLoanScale and never outgrow STAmount's 16 digits, so
  the promotion round-trips losslessly. Also drops the misleading
  "vault-associated" descriptor (the underlying fields live on the
  loanSle).
- LoanRounding_test.cpp: the "loan scale ~ -13 vs vaultScale ~ -9" note
  is impossible under computeLoanProperties (which clamps loanScale to
  max(vaultScale, amount.exponent())). Rewrite to describe the actual
  loanScale == vaultScale regime the scenario exercises.

src/libxrpl/tx/transactors/lending/LoanManage.cpp

@@ -191,9 +191,10 @@ LoanManage::defaultLoan(
     // sibling change in LoanPay for the same fix on the payment path.
     //
     // The Number-precision guard `T - A >= totalDefaultAmount` below is
-    // exactly sufficient: loanVaultExposure returns a difference of vault-
-    // associated STNumber fields (IOU-normalized via associateAsset), so
-    // its STAmount promotion round-trips losslessly with no writeOff slack.
+    // exactly sufficient: loanVaultExposure returns a difference of
+    // STNumber fields that are whole multiples of the same immutable
+    // `10^sfLoanScale` and never outgrow its 16 digits, so its STAmount
+    // promotion round-trips losslessly with no writeOff slack.
     bool const useUnifiedAssetArithmetic = view.rules().enabled(fixCleanup3_4_0);
 
     if (useUnifiedAssetArithmetic)

src/libxrpl/tx/transactors/lending/LoanPay.cpp

@@ -445,10 +445,14 @@ LoanPay::doApply()
     // normalization is applied symmetrically and the two balances land on
     // the same STAmount value -- which is what makes the debug invariant
     // sfAssetsAvailable == pseudo-account balance hold. Pre-amendment,
-    // sfAssetsAvailable was fed the vaultScale-rounded value while the
-    // pseudo-account received the raw Number, creating the same
-    // cross-side asymmetry with `assetsTotalDelta` that
-    // `LoanManage::defaultLoan` addresses on the default path.
+    // sfAssetsAvailable and the pseudo-account both received the
+    // vaultScale-rounded value (accountSendMulti was called with
+    // `totalPaidToVault`, i.e. the rounded amount), but sfAssetsTotal
+    // was mutated with the unrounded `assetsTotalDelta` on the very
+    // next line. That vault-side asymmetry between sfAssetsAvailable
+    // (rounded) and sfAssetsTotal (unrounded) is the counterpart of the
+    // cross-side mismatch `LoanManage::defaultLoan` addresses on the
+    // default path.
     auto const totalPaidToVault = view.rules().enabled(fixCleanup3_4_0)
         ? totalPaidToVaultRaw
         : roundToAsset(asset, totalPaidToVaultRaw, vaultScale, Number::RoundingMode::Downward);

src/test/app/lending/LoanRounding_test.cpp

@@ -257,9 +257,17 @@ private:
             env(pay(issuer, borrower, asset(5'000'000'000)));
             env.close();
 
-            // Vault magnitude ~1e7 so vaultScale ~ -9; the loans below sit at
-            // loan scale ~ -13. That 4-order gap is where storage precision
-            // can differ from loan-tracked Number precision.
+            // Vault magnitude ~1e7 gives a coarse vaultScale; the loans
+            // below use principals sized so that
+            // `computeLoanProperties(..., minimumScale=vaultScale)` clamps
+            // `loanScale = std::max(vaultScale, amount.exponent())` down
+            // to `vaultScale`. That equal-scale regime is precisely the
+            // one where the pre-amendment paths still round vault-side
+            // mutations asymmetrically (rounded on sfAssetsAvailable and
+            // in the pseudo-account transfer, unrounded on sfAssetsTotal
+            // via assetsTotalDelta / defaultCovered), so the dust-snap
+            // and post-rounding invariant branches this test guards
+            // against get maximum exposure.
             BrokerParameters const brokerParams{
                 .vaultDeposit = Number{10'000'000},
                 .debtMax = Number{0},