Document in the C++ stdin/stdout host that it's UTF-8

[jonwis]

The C++ process pipeline handler (the side that creates the child process) properly converts its written content to UTF-8 (via winrt::to_string) but it hands a std::string back to the caller from the output. The docs on that should say "this is a UTF-8 string smuggled in a std::string" ... There is no C++ standard support for UTF-8, but winrt::to_hstring(std::string_view) exists and does the right thing now. microsoft/cppwinrt#269 (comment)

To do:

• Document that the process pipe thinks in UTF-8 and the std::string is UTF-8 null terminated
• Show an example of converting from std::string to std::wstring for use with normal string operations

To consider: should there be a specific new type that is "I am a smuggled UTF-8 string" so you have to do something with it before operating on it unless the dev knows what they're doing?

[jonwis]

Fixed