C++: Add QLDoc for getOutputArgument

Author: Robert Marsh

cpp/ql/lib/semmle/code/cpp/commons/Printf.qll

@@ -255,7 +255,10 @@ class FormattingFunctionCall extends Expr {
   }
 
   /**
-   * 
+   * Gets the argument, if any, to which the output is written. If `isStream` is
+   * `true`, the output argument is a stream (that is, this call behaves like
+   * `fprintf`). If `isStream` is `false`, the output argument is a buffer (that
+   * is, this call behaves like `sprintf`)
    */
   Expr getOutputArgument(boolean isStream) {
     result = this.(Call).getArgument(this.(Call).getTarget().(FormattingFunction).getOutputParameterIndex(isStream))