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>