syscall: update comments for x/sys migration

The comments in this package state that users should be
migrating code that uses the syscall package to its
corresponding package in x/sys. However, the syscall.Signal
and syscall.Errno types and the syscall.SysProcAttr struct is
not defined in the x/sys package and still need to be referenced
from within syscall.  This adds a change to the comments to
clarify that the migration will need to continue to use some
references to syscall for now.

Fixes #19560

Change-Id: I8abb96b93bea90070ce461da16dc7bcf7b4b29c1
Reviewed-on: https://go-review.googlesource.com/39450Reviewed-by: Rob Pike <r@golang.org>

src/syscall/syscall.go

@@ -22,7 +22,10 @@
 // Go repository should be migrated to use the corresponding
 // package in the golang.org/x/sys repository. That is also where updates
 // required by new systems or versions should be applied.
-// See https://golang.org/s/go1.4-syscall for more information.
+// Signal, Errno and SysProcAttr are not yet available in
+// golang.org/x/sys and must still be referenced from the
+// syscall package. See https://golang.org/s/go1.4-syscall
+// for more information.
 //
 package syscall