C++: Improve the SideEffect library QLDoc.

cpp/ql/src/semmle/code/cpp/models/interfaces/SideEffect.qll

@@ -18,14 +18,18 @@ abstract class SideEffectFunction extends Function {
   /**
    * Holds if the function never reads from memory that was defined before entry to the function.
    * This memory could be from global variables, or from other memory that was reachable from a
-   * pointer that was passed into the function.
+   * pointer that was passed into the function. Input side-effects, and reads from memory that
+   * cannot be visible to the caller (for example a buffer inside an I/O library) are not modelled
+   * here.
    */
   abstract predicate hasOnlySpecificReadSideEffects();
 
   /**
    * Holds if the function never writes to memory that remains allocated after the function
    * returns. This memory could be from global variables, or from other memory that was reachable
-   * from a pointer that was passed into the function.
+   * from a pointer that was passed into the function. Output side-effects, and writes to memory
+   * that cannot be visible to the caller (for example a buffer inside an I/O library) are not
+   * modelled here.
    */
   abstract predicate hasOnlySpecificWriteSideEffects();