windows: comment on fields omitted from the win32finddata1 struct

Updates golang/go#42637

Change-Id: I2e8c95af3cf2172c55bda6a15a749afcc6c45581
Reviewed-on: https://go-review.googlesource.com/c/sys/+/284172
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Trust: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

windows/types_windows.go

@@ -794,6 +794,14 @@ type win32finddata1 struct {
 	Reserved1         uint32
 	FileName          [MAX_PATH]uint16
 	AlternateFileName [14]uint16
+
+	// The Microsoft documentation for this struct¹ describes three additional
+	// fields: dwFileType, dwCreatorType, and wFinderFlags. However, those fields
+	// are empirically only present in the macOS port of the Win32 API,² and thus
+	// not needed for binaries built for Windows.
+	//
+	// ¹ https://docs.microsoft.com/en-us/windows/win32/api/minwinbase/ns-minwinbase-win32_find_dataw describe
+	// ² https://golang.org/issue/42637#issuecomment-760715755.
 }
 
 func copyFindData(dst *Win32finddata, src *win32finddata1) {