C++: QLDoc for FileClosed, LoopBounds and MemoryFreed

cpp/ql/src/Critical/LoopBounds.qll

@@ -2,12 +2,23 @@
 
 import cpp
 
+/** An assignment to a variable with the value `0`. For example:
+  * ```
+  * int x;
+  * x = 0;
+  * ```
+  * but not:
+  * ```
+  * int x = 0;
+  * ```
+ */
 class ZeroAssignment extends AssignExpr {
   ZeroAssignment() {
     this.getAnOperand() instanceof VariableAccess and
     this.getAnOperand() instanceof Zero
   }
 
+  /** Gets a variable that is assigned the value `0`. */
   Variable assignedVariable() { result.getAnAccess() = this.getAnOperand() }
 }