C++: add QLDoc to side effect functions

Author: rdmarsh2

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

@@ -29,12 +29,24 @@ abstract class SideEffectFunction extends Function {
    */
   abstract predicate hasOnlySpecificWriteSideEffects();
 
+  /**
+   * Holds if the value pointed to by the parameter at index `i` is written to. `buffer` is true
+   * if the write may be at an offset. `mustWrite` is true if the write is unconditional. 
+   */
   predicate hasSpecificWriteSideEffect(ParameterIndex i, boolean buffer, boolean mustWrite) {
     none()
   }
 
+  /**
+   * Holds if the value pointed to by the parameter at index `i` is read from. `buffer` is true
+   * if the read may be at an offset. 
+   */
   predicate hasSpecificReadSideEffect(ParameterIndex i, boolean buffer) { none() }
 
   // TODO: name?
+  /**
+   * Gets the index of the parameter that indicates the size of the buffer pointed to by the
+   * parameter at index `i`.
+   */
   ParameterIndex getParameterSizeIndex(ParameterIndex i) { none() }
 }