C++: expand a comment

Author: Robert Marsh

cpp/ql/src/semmle/code/cpp/rangeanalysis/RangeAnalysis.qll

@@ -161,7 +161,11 @@ private IRGuardCondition boundFlowCond(ValueNumber vn, NonPhiOperand bound, int
 {
   exists(int d |
     result.comparesLt(vn.getAUse(), bound, d, upper, testIsTrue) and
-    // strengthen from x < y to x <= y-1
+    /*
+     * `comparesLt` provides bounds of the form `x < y + k` or `x >= y + k`, but we need
+     * `x <= y + k` so we strengthen here. `testIsTrue` has the same semantics in `comparesLt` as
+     * it does here, so we don't need to account for it.
+     */
     if upper = true
       then delta = d-1
       else delta = d