Improve httpErrors existingResponse documentation

[mcm-ham]

As evidenced by this question and answer the current documentation is a bit confusing. https://stackoverflow.com/a/21271085/222748

It also seems there is a missing argument fTrySkipCustomErrors to this method in the docs: https://docs.microsoft.com/en-us/iis/web-development-reference/native-code-api-reference/ihttpresponse-setstatus-method

I tried downloading the C++ IIS docs to check, but the "The latest Windows Vista Platform SDK is available here" and "The source code for this module is available in the Visual Studio IIS7 Native Module Sample" links on this article do not work. https://docs.microsoft.com/en-us/iis/develop/runtime-extensibility/develop-a-native-cc-module-for-iis