bsip38: rewording; add max_debt_to_cover formula

bsip-0038.md

@@ -82,6 +82,14 @@ max_amount_to_sell = (debt * target_CR - collateral * feed_price)
 
 The result is a rational number, need to be rounded up to an integer.
 
+Then, the maximum debt it wants to cover can be calculated as:
+
+```
+max_debt_to_cover = max_amount_to_sell * match_price
+```
+
+It need to be rounded up to an integer as well.
+
 
 # Specifications
 
@@ -143,14 +151,14 @@ The rule for matching a limit order with a call order need to be revised as
 smaller and the short position will be closed after the match, round up its
 paying amount;
 * **otherwise,**
-  * **if `target_collateral_ratio` is set and it's receiving the maximum debt
- amount calculated with `target_collateral_ratio`, see this order is smaller,
- try to round up its paying amount;**
-    * **for edge cases, if its collateral ratio would not increase after being
-   partially filled due to the round-up (which may even cause a black swan
-   event in an extreme scenario), see `target_collateral_ratio` as not set for
-   this time, re-apply the filling rules for this match.**
-  * otherwise, round down its paying amount.
+  * **if the call order has `target_collateral_ratio` set and is receiving the
+ maximum debt amount calculated with `target_collateral_ratio`, see the call
+ order as smaller, try to round up its paying amount;**
+    * **for edge cases, if the call order's collateral ratio would not increase
+   after being partially filled due to the round-up (which may even cause a
+   black swan event in an extreme scenario), see its `target_collateral_ratio`
+   as "not set" for this time, re-apply the filling rules for this match.**
+  * otherwise, the call order is larger, round down its paying amount.
     * if the limit order would receive nothing, cancel it (it's smaller,
    so safe to cancel);
     * otherwise, calculate the amount that the limit order would pay as