cmd/link: add TODO missing from CL 70310

CL 70310 dropped TODO while moving code.
Add TODO back, so we do not forget.

Change-Id: I3599ac02743bd35fb9556fdc238e9c72cf7f718f
Reviewed-on: https://go-review.googlesource.com/71590Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/link/internal/ld/pe.go

@@ -93,6 +93,7 @@ const (
 
 // TODO(crawshaw): add these constants to debug/pe.
 const (
+	// TODO: the Microsoft doco says IMAGE_SYM_DTYPE_ARRAY is 3 and IMAGE_SYM_DTYPE_FUNCTION is 2
 	IMAGE_SYM_TYPE_NULL      = 0
 	IMAGE_SYM_TYPE_STRUCT    = 8
 	IMAGE_SYM_DTYPE_FUNCTION = 0x20