syscall package: document that errno is zeroed on success

syscall package: document that errno is zeroed on success

This is a documentation enhancement only, without any code
change.

The rationale for documenting this precisely is that Unix
programmers who "know" that errno's value is undefined after
a successful system call may be surprised otherwise and
search to be sure that a zero errno may be relied upon after
successful calls.

R=r, rsc1, rsc
CC=golang-dev
https://golang.org/cl/812044