Preprocessor: document that removeComments() may throws exception.

No code change.

src/preprocessor.h

@@ -95,6 +95,7 @@ protected:
      * Remove comments from code. This should only be called from read().
      * @param str Code processed by read().
      * @return code without comments
+     * @throws std::runtime_error when code contains unhandled characters
      */
     static std::string removeComments(const std::string &str);