x/sys: improve NewCallback documentation
x/sys: improve NewCallback documentation

Keeps the documentation in sync with syscall package. Copies the changes from CL https://golang.org/cl/126035.

Change-Id: I778725fd7d5e96ddf839d33e1d220e5a12e2d24f
GitHub-Last-Rev: 4b30513126d7ffbca31640a6120a2259b6a226db
GitHub-Pull-Request: golang/sys#12
Reviewed-on: https://go-review.googlesource.com/126435
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

golang/sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys