Implement better commenting in AppInstance.cpp's GetCommandLine.

[aeloros]

Probably should have a comment somewhere saying what the command line we're trying to parse actually is. I'm guessing

// The payload begins with four dashes and a contract name (no spaces allowed),
// optionally followed by a colon and data (no spaces allowed).
// The payload ends at the next space or end of string.
//
// Example:
//  contoso.exe blah ----somecontract:eW91IGFyZSBjdXJpb3Vz blah
//                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
//
// If there is no data, then an empty string is returned as the data.
//
// Returns { contract_name, data } if a payload is found.
// Returns { L"", L"" } if no payload is found.

Originally posted by @oldnewthing in #823 (comment)