syscall: comment on fields omitted from the win32finddata1 struct

Updates #42637

Change-Id: I4c7d38034b60c2c04efdeb530a97d96deddfd6fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/284152


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>